No it doesnt.
If the boat doesn't have salt water on it, and presumably hasn't been in the water, that would blow away the theory that any body was dumped at sea.
Whose theory? Your theory.
If a body was dumped at sea, and he cleaned it up, why would they want to make a big deal out of no salt water being found?
It's not about whether the boat was or was not in the water at the Marina that day. It's about whether sp's alibi holds true or not. If the boat shows no evidence of salt water, this means that SP lied about his boat being in the water at the Marina.