None. Nothing special about an engine just because you stick it in a boat.
One of the issues is being on the water means a lot of humidity in the air. Ethanol absorbs water. In high humidity areas the "breathing" of a fuel tank with temperature changes will bring water into the fuel.
https://www.epa.gov/sites/production/files/2015-09/documents/waterphs.pdfSince ethanol and water readily dissolve in each other, when
ethanol is used as an additive in gasoline, water will actually
dissolve in the blended fuel to a much greater extent than in
conventional gasoline. When the water reaches the maximum amount
that the gasoline blend can dissolve, any additional water will
separate from the gasoline. The amount of water required (in
percent of the total volume) for this phase separation to take
place varies with temperature, as shown in Figure 1. As an
example, at 60 degrees F, water can be absorbed by a blend of 90%
gasoline and 10% ethanol up to a content of 0.5 volume percent
before it will phase separate. This means that approximately 3.8
teaspoons of water can be dissolved per gallon of the fuel before
the water will begin to phase separate.