Lets try troubleshooting , in a logical manner
Its very likely not firing on all cyls, on the hose it freewheels the dead cyl's, in the water it can't.
Go to the auto store, buy a compression guage and an adjustable spark tester ( not the neon type), should cost you less than $50 total.
Inspect sparkplugs, keep them in the order they came out, describe what you see.
Compression test it, all plugs out, fully charged battery. Write the results down.
spark test it, all plugs out, set tester gap to 3/8th inch. Look for BLUE spark, yellow is fail.
Heres a vid, this guy has a 6 cyl tester, it checks them all at the same time, see the blue spark and notice the huge gap the tester is set at, you cannot test for spark with a sparkplug because the required test gap is so big.
if you find no problems rebuild the carbs with new carb kits.
Theres lots of videos on Youtube for DIY carb jobs, its very simple.
Heres a mid 80's Johnson 115hp carb rebuild video.