Given the rules, it would be a better experience if squares only have two states: ship block or empty. There is no point to letting the user draw water. The feature probably exists with the intent of letting users mark squares that can't possibly be ships, but why wouldn't the game do that step by itself based on the blocks placed? That would make the game a lot less tedious.
It's indeed the way it is so that you can indicate a square that must be water. What you propose sounds like a solver to me, and I much prefer solving it myself.
Note that very often, the solution process for a more difficult puzzle involves going through nontrivial logic to prove that a particular square must be water. It's important to be able to record non-obvious "must be water" squares.
If you do it as a purely visual representation of the game state, it won't be a solver, will it? Just all blocks surrounding ships + blocks in rows/cols with a 0.
