Some will argue.----I would charge the battery.----Then have battery load tested ( even if new ) last week.-----Then take starter apart for inspection .----Test ohms on armature.----No money spent on these tests !
I am well aware of how much it takes to drive a boat at 40 MPH !!!----Point is folks drain out very good gear oil on the outdrive and rarely even check rear axle on the pick-up truck !
Agreed with 20 hrs a season gear oil can be used for many seasons.-----After all many rear axles in pick-up trucks rarely get oil changed for years and years..----If there is no water in the gear oil it is good for another season.
Did you clean carburetors ???----Did you install latest water pump kit with instruction to move tell tale fitting to top of block.----And other cooling modifications , yes or no ?
I doubt it was the rubber parts.-----I think rust from crankshaft jammed the drive shaft in the crank.----A very common thing on motors that get little or no loving care !!
Fancy carpet and upholstery hides a lot of things.------I would not venture out in a " GREW " without inspecting the floatation foam under the floor.---