How much is enough
If focusing solely on the HP part of the equation, 21 or similar should move the boat with no problem. We found that the 23 HP diesel factory-installed on our 10600# boat was just fine, for decades.
I replaced it with a 25 HP engine because that was close enough in the model lineup these days, and also because I was spec-ing out a replacement with a larger (70 A) alternator. (Alternators use up some measurable HP when charging.)
BTW, if pondering a change sometime, please have a look at my blog entries on our engine change.
Great filter upgrade in your picture -- I also upgraded to a Racor 500 model when changing out the engine.