I would take it back. I'm not trying to be mean, or offend you, so please don't take it that way, but it doesn't sound like your mechanically inclined enough to try and figure out the problem on your own, beyond some very simple things, which isn't a bad thing or a negative on you as a person, it's just that you could potentially do more damage than good.
A couple of things you can check are
Battery connections- make sure they are tight and clean
Fuel filter/s- If your boat is equipped with them, you may have a clogged one
fuses, if your boat has any
In general, a motor needs gas and spark/electrical to run. If it turns over but does not start it's missing something in that equation. Now, it could be something as simple as a fuse or relay, but that kind of thing needs to be traced out.
You said that the mechanic set the timing and fixed the starter. If the engine turns over I would say the starter is ok. If you just hear a whining sound and the motor doesn't turn over, then it's a starter problem. The fact that he changed the timing could be the problem if he didn't get everything tightened down or something changed with it.
The problem could be related to what the mechanic did, or it could be something else. Unless you want to try and trace the problem down yourself, I don;t think you have a choice but to take it back. It might turn out that the problem is something they did while they had it the first time.