Do one's own provinces trade with each other? I have a vague recollection that I read a post somewhere that said they don't, but I cannot recall where....

If this is the case, I imagine that invading a trading partner province would also act to reduce the influx of florins...In my current campaign (as Sicilians - thanks to whoever it was who suggested them, I'm having a ball) I have just taken Tripoli and Palestine from the Eggies. My trade has increased, but this could be the result of actions elsewhere on the map.

As for "pleasure boats", yes that is a major pain, especially around choke points as Sensei Warrior mentioned - and even more so if you are a Cath faction and your pleasure cruising antagonist is another Cath faction....because taking appropriate action could result in old mate Popie beating you with the excom stick.