To me the answer is simple, get the Tiger Shark. I own a Tiger and love it. My thought train is that if you are gonna be doing water detecting, you need something dependable and new with a warranty. That is why I purchased a brand new Tiger Shark. Now if you think you are gonna just do water detecting occasionally, get a used rig. If buying used I would still stick with a newer Tiger Shark. Water goodies are not as plentiful as they used to be in freshwater. There is a another detector company that has made a black water tight version of an Ace 350 and a lot of people have them and are now water hunting. I personally like a dedicated water detector, but that's just my preference.
One thing I can tell you is that on land, hunting in all metal with background threshold audio, this detector is DEEP! Doing this it can hit all the targets at 12" in my less than 2 year old test garden. Most machines struggle on the 6" targets in this fresh test garden.
I like the 10" coil. You get more coverage and better depth. Target separation in the water is a mute point- you're gonna use a sand scoop and scoop it all anyways. By the way, you better plan on purchasing a good quality stainless steel hand scoop. I use a T-Rex and can highly recommend it. An angled handle arrangement like the Stealth or T-Rex with a floating wood handle is my recommendation.