I think you need to apply Occam's Razor and shave the stuff off around your prop.
That means, rule out the most likely cause before assuming anything more complicated. The simplest cause is a prop fouled by seaweed, which fits every symptom and which you actually observed.
The second simplest cause here is something misaligned between the rotor, cap, and ignition wires. You could be missing on one or more cylinders.
The
dinghy rode incident doesn't sound like the problem.