The problem with adding forced induction in a marine application is exhaust manifold cooling capacity. Adding a turbo will significantly raise EGT's, and water cooling that exhaust is NOT an easy task for a fabricator. Aside from building a log style manifold, and "boxxing it in", getting a true tubular manifold will be difficult.
The second issue is fueling and timing control. I know the 1.8L engines have that capability, but I'm unsure in the TR-1's. Even if you do, you need some good datalogging and tuning software, as well as the appropriate instruments on the engine to adjust fuel and air. Things like O2 sensors won't live long in water cooled exhaust manifolds.
A final hiccup, is the remaining overhead in engine performance. I suspect there is SOME in these engines, however whatever you increase in power you will decrease in engine life. The Yamaha engines are proven to be VERY reliable, and I'll take the hit on performance in exchange for the reliability.
With all that said, I've often thought about getting a low cost/possibly damaged SX190, and throwing a turbo, larger injectors and pump, and an aftermarket ECU on there to see what I could get done. My thought is the limiting factor will be the pump size and impeller pitch. The 195's moved to a 160mm pump for this reason.