2) How long would it take to notice a temp change in the water comming out of the hotwater tap? (tank holds about 20L)
It depends how much waste heat your engine is generating, which depends on the load.
On my boat, if you're sitting at the dock
with the engine idling, the answer is "about forever". On the other hand, I can have dangerously hot water after motoring into an anchorage and dropping the anchor
-- an hour maybe? I've never really tried to determine the minimum time.
(I say dangerously hot because the water in the tank eventually rises to the temperature of the engine coolant, which is much hotter than you would want to get on your skin. We are always careful not to turn the handle all the way to "hot" when using engine-heated water.)
The important point is that the engine isn't making much heat unless it is working hard. If it doesn't make much heat, there isn't much heat to dump into your water heater. There also isn't much need for coolant flow. My engine can idle for a long time with unreasonably low coolant levels, but not overheat. The alarm
comes up 5 or 10 minutes after I leave the dock
If you go out an motor around for a while, I expect one of two things will happen: either your engine will overheat (because of the air lock resisting coolant flow) or you will get hot water from your tap. Instead of motoring, some people put the engine in gear
and raise the throttle while still tied to the dock. This makes the engine work harder, but some marinas
don't want you doing it because of the strain it puts on cleats/pilings/etc.