You can point out water through naked eyes by simply checking your oil-water separator equipment installed just beside the engine. When water settles excessively into the virtual separator you need to eliminate it immediately from the system. Although a small amount of water won’t make any major effects on the engine. Still, you have to inspect it on regular basis (probably when you do refueling).
The problem with water in boat gas tanks is that it can lower the adequate lubrication which can practically hurt other mechanical components inside the engine. For instance, if you own angler gas tanks made from aluminum then it is very rare that you found any damage at the inside layers of the tank. But as I interpreted before, due to a small amount of water distribution in the fuel you should always opt for aluminum fuel tanks.
Instead of disposing water manually from canister it would be a better choice to deploy an additional paper fuel-water filter element. It will prevent any dilute to enter into the engine pipelines if the other separators fail to work.
Boat gas tanks are not an ideal choice for modern cruisers as they prefer to go with fuel systems. Most boats releasing nowadays are having one permanent and one auxiliary tank. If you own an old fishing vessel then you might need to replace a tank. Remember that water can create corrosion and it can create friction that causes routine components to operate harder than usual.
The rust building affects welds of the angler gas tanks and the consequences can be threatening amidst your way. You need to contact experienced manufacturers who know to build storage tanks with minimum welds. Or in case you still didn’t find the right way of pouring out from the separator, you should consult to a skilled professional. There can be other mighty sources that can root of water drift so in that content see a marine accessories store or tank builder.