Re: 98 7.4L MPI manifolds/risers
the boat doesn't heat up when WOT. should I change them or do most people wait until the boat runs hotter than usual?
Thermostat controls minimum temp.
Sounds like you have a "little bit of money" to spend right away.
Manifolds and risers
BEFORE they fail and destroy the engine.
Full drive service, to include - bellows(all 3) raw water impeller, gimbal bearing(maybe, I would do it since you don't know history).
Thermostat
Full tune up, plugs, cap, rotor, wires(?), Fuel filter/s......