I've been told that a larger engine would'nt be working as hard and could be
as economical as the smaller 2.3ltr.

Anybody with that much excess bologna should open a deli.

Also,would a larger engine last longer if serviced propably as it would'nt
be working so hard.

Sounds like you like to run close to WOT nearly all the time. You stated that
you are disappointed with the WOT speed of your boat and want to go much
faster, if possible. With that kind of use, expect the probable lifespan of
most gasoline engines to be about 1000 hours, tops, and could be less. That
larger engine is usually pushing a larger and heavier boat, so it's tough to
say it isn't "working as hard" as a smaller engine pushing a lighter boat.

Speed vs. fuel economy? Expect to lose economy much faster than you pick up
speed, once you have your boat up on plane. The relationship does not graph as
a straight line, but is almost always represented as an extremely steep and
accelerating curve.