Are there any ways to save on fuel in boats?

Hoist the sail! Much quieter, sometimes faster and uses no carbon based fuels.

Being a sailor, I agree with the first answer.

There are some electric drives coming on the market. They require some additional weight in batteries, but the engine is small and lightweight.

What’s expensive, in power boats, is going fast, in other words, planing. Some boats have displacement type hulls, which will never plane and whose upper speed limit is a factor of the waterline length of the hull. These can be operated on much less fuel, but won’t achieve planing speeds.
Keelboat sailboats are usually displacement hulls as are most work and commercial fishing boats. Trawler yachts, for instance, are cheaper to run than cabin cruisers. The former are typically displacement hulls and the latter are usually planing hulls. And trawler yachts are more seaworthy.

Break out the ours and hope for full sails!