Initial timing at idle isn't very important in a jet boat application.  I'd recommend 32-34 degrees all in at 2500-2600rpm.  Depending on how much mechanical advance your distributor is set up to provide will determine where the initial falls.  As long as you don't have any issues with starting or run on when shut down you should be good to go.

If you're feeling racy you can try kicking it up a couple degrees at a time until you start to detonate (just don't hit the nitrous  ).