The boat has a Beta Marine 10 with 830 hours. When turning the starter key you hear the solenoid's click but the starter doesn't crank. Any suggestions on what to check for?
You need to determine whether the starter is faulty of if the engine is the cause of the problem.
Remove the starter and try it on the workbench.
If the starter does not work, take it to a repair shop for alternators and starters.
If the starter does work properly on the bench you most likely have backfilled the engine with water through the exhaust. If this is so, re-install the starter, drain the muffler and take out the glow-plugs and crank the engine over. If it turns ok with no noises, refit the glow-plugs and start the engine.