Fisher is an excellent choice, what you get is extremely long battery life (75 hours -- and they really do it) and aural target IDs. Depth to 250 feet. No extra short shaft to buy or "scuba mate" attachment to convert it to scuba use, it's ready to go for land or sea from the box. I have a couple Fishers, and I like the Fisher equipment.
The Fisher, set up for diving appears here:
http://www.kellycodetectors.com/fisher/1280.shtml A negative with the Fisher is you have to know which coil you want from the get-go. I select the large coil (as shown) on the Fisher and Minelab since I want to cover the maximum amount of ground I can with the limited bottom time I've got.
http://www.kellycodetectors.com/fisher/fisherwatersurfland-1280x.htm -- 1280 with large coil is at the bottom Ground balances (tunes out salt, other anomolies) as you hunt.

The Tiger Shark has the features above but reduced battery life (12 hours) and reduction in depth by 50 feet. It's weak point is replacing the batteries means opening the case and exposing the circuit board to the elements, whereas the Fisher/Minelab/etc. keep the circuit board and the drop in battery pack seperate. It has a lifetime warranty, although it is not warranted if water is detected on the circuit board. A positive is it has a 7 1/2 and 10" coil that can be changed easily. Some manual ground ballancing required, so you'll need to understand tuning out of various soils (not hard -- and a plus if you're looking for nuggets out west in magnetic iron soils).
http://www.tesoro.com/Manuals/TigerShark_Manual.htm
http://www.dixiemetaldetectors.com/tesorotiger.htm

Although double your price range, I'm including a Minelab Excalibur page and comment for someone else: http://www.kellycodetectors.com/minelab/minelab.htm The Excalibur is the choice for gold hunting. Negatives are the battery pack must be recharged (no drop in batteries) on a wall or ciggarette unit, all the accessories are expensive, the coil size must be known before you buy it, and the wires are somewhat delicate where the headphones fit into the unit and it is not suited to land no matter what they say on the sales site. With all that said, why would you buy it: because it will find the little tag that says "14K" that is on the clasp of a gold chain that has been lost . . . It is that sensitive and that good. The rest of the metal detectors are good too, they're just not THAT good. Over time, the difference in price is negligable (if you use it), since the Minelab will find the small rings and pieces of broken chains and jewelry (scrap gold) that the others will typically miss. An option out west (where you have a lot of hot rocks, mineralized ground, etc.) is the Garrett Infinium: http://www.kellycodetectors.com/Field%20Test%20Reports/garrett/GInfiniumLS2Fieldtest.htm (towards the bottom is the test on nuggets) for $999.

Garrett Sea Hunter: I like the Sea Hunter, my old unit has lasted over 20 years. You can switch from small to large coils easily by simply unscrewing the coil from the box. The difference between the SH II and my old unit is batteries, the older units came with nicads you had to send back to the factory every 5 years. As a result, there's a lot of old Sea Hunters sitting around with a dead set (mine included). It's just to easy to pick up a Fisher and go hunting for 50+ hours . . . With the Sea Hunter II, you now have drop in batteries so it's like other units (20 hours). I suspect, with the smaller coil, it will be just as effective on nuggets underwater as the unit above. These are pulse units, if you understand sonar, you kind of get the idea. It will ignore mineralized ground, salt, etc. The negative is it won't pick up the 14K tag on the chain above, but is offset in that it will detect a cannon ball at 12 feet under the soil with even the standard coil. Proffessionals have used them reliably for many years. At the dawn of Pulse Induction Sea Hunters, the difference in price between it and the next unit that would do what it did for $500 was $9500 . . . so it wasn't brain surgery to outfit a whole dive team with Sea Hunters. Mine paid for itself and a couple other detectors ($2000 worth of treasure) the very first day I had it, so it will always have a special place in my heart, whether I'm using it or not. Andy did a pretty good test on one here: http://www.kellycodetectors.com/Field%20Test%20Reports/garrett/GSeaHunterMark2Fieldtest.htm I will probably buy one of the new Sea Hunter II's myself, with the small coil, and use it in the rivers out west for nuggets this year. I could tell you a lot more after next year for its reliability for prospecting use. I typically use the small coil with this unit anyway, since pulse induction means it goes extremely deep, and the small coil comes in very handy in some of the tight places while diving, and you can take a snap-ring and snap it to your BC and it hangs unobtrusively with the small coil.

So -- I didn't give you an answer -- just more choices (as it should be). I've pulled info from a lot of places to buy these things as well, but you should beware of Ebay deals. There have been cases of people who became distributors with the intent of stocking up quickly and going out of business and bankrupt and sell off the loot on Ebay. If someone is offering lower prices than anyone else can match, it might not be for "friendship" -- watch yourself! These scams hurt us all ultimately as the prices go up (usually for the accessories you need is where the increases go) every time these scam artists pull these things on the distributors.

GOOD LUCK OUT THERE!!!!!! You are enbarking on one of the most fun and profitable endeavors you can have no matter what you choose -- underwater metal detecting.

Matt

If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen.
-- Samuel Adams, speech at the Philadelphia State House, August 1, 1776.