Locks might otherwise be useful for practising jumps? Similar speed limit to rivers... I've skied up the Ouse a long time ago, fishermen wave and you wave back lol...

I still need to pull my finger out and see if I can fix my old speedboat's 3cyl Mercury 70hp 2 stroke engine circa 1982. I rebuilt the powerhead, rebore and new pistons (a pain because the cyl head isn't removable so pistons go in from the crank end and the design is such they're all supposed to go in at the same time while attached to the crank - without the special tool I managed 2 at the same time attached to the crank then attached the 3rd bottom end with the crank just off the main bearings), rebuilt the 2 carbs (yes it's a 3 cyl with 2 carbs), yet it still hardly ever runs right. 2 Stroke fumes didn't bother me much as a kid, they do now, in fact I don't like anything about 2 strokes and especially not messing about with fiddly needle bearings... I've thought about ripping the powerhead off (again) and seeing if I can graft on a 4 stroke car engine instead!

Have seen a few inboard engine'd boats with engine coolant to raw water heat exchangers (engine runs pressurised coolant similarly to a normal car) but I think raw water still runs through the marine special exhaust manifold. Marine exhaust manifolds are another rot spot, not far off the price of an engine and are more difficult to pick up second hand than engines.

Cast iron welding specialist?