If you have any acid spill, a fiberglassed bottom would contain it but otherwise your design looks good.
Boats have very high standards for construction, here are some.
Batteries Sec. 183.420 Batteries
Each battery must be installed so that metallic objects cannot come into contact with positive terminals.
(Or the negative terminal if you use a positive ground.) How can this be accomplished?
Boots on the positive terminal.
An insulating cover over the terminals.
There are other ways. The whole point is that you do not want a wrench or screwdriver of something metal that falls on the battery (like a metal portable gas tank) to come in contact with the positive terminal. This could cause a large arc and possibly a fire or explosion. It is best avoided.
There must be a means to vent hydrogen gas from the batteries to the atmosphere.
Hydrogen gas is extremely flammable. The slightest spark will set off an explosion. Hydrogen gas discharged by a battery during charging must not be trapped in the boat, but free to make it's way out of the compartment to the atmosphere. Make sure the battery compartment is ventilated. If you put the battery in a box, make sure the box is vented and the gas has a way to escape from the compartment where the battery is located. Hydrogen dissipates very rapidly. A simple hole in the top of the compartment will work.
With the proliferation of sealed batteries some people have asked, well, if the battery is sealed why do I have to ventilate the battery? Because sealed batteries can vent hydrogen. They are what is known as Sealed Valve Regulated or SVR batteries. The valve in each cell is there to vent hydrogen if the battery overheats or overpressures. Under normal operating conditions they don't outgas, but they can. The space that the battery is in needs to be vented.
The battery must be secured so that it can't move around. That means it can't move more than one inch forward and aft, sideways, or up and down.
A battery box will help but then the box has to be secured. Most battery boxes or trays come with a strap to hold them down. If not secured batteries can tip over, slide around, and spill acid. In one case a battery tipped over and made contact with a metal fuel tank. After the resulting discharge of electricity and fire the battery studs were welded to the tank. Batteries must be secured.