That's not such an unreasonable price.
I would assume the the dealer is quoting for a marine alternator to keep the boat safe and legal, as to do otherwise would expose him to liability issues. Such an alternator would cost $200 or more.
As always, you can save a lot of $$$ doing the labor yourself. Changing the alternator is not hard, as long as you aren't trying to change how it mounts, the belt arangment, etc. I would consider it to be about twice as much work as just changing the belt.....
My best recommendation is to not get cheap: buy a MARINE alternator. There is a difference, the marine unit will be "ignition protected" so that it won't blow your boat to bits if you happen to get a gas leak. The Coast Guard requires the use of marine units for just about everything in the engine compartment, and your insurance company would not be happy if they investigate a claim and found non-marine components.