I've got a 175amp alt on a 1 cyl kubota engine. Water cooled, lived under my cockpit floor behind my main engine. Bit of a bear to get to to work on, but runs great. It basically runs at max RPM, may switch it to use a variable speed controller like they have on airplanes(push/pull/twist for fine adjustment). Bugger vibrates like no other but she was cheap, pushes out 110amp continuous without any troubles. It does seem like that 110amps is the sweet spot for the engine before its bogged down. Eventually I'll upgrade to lithium and will futz with the output a bit more to refine it.
Would highly recommend belt driven alts on an engine regardless if its on a boat or not. Works great, simple and easy to work on and maintain.