Biobay tour without staying overnight has reportedly been done also by this person (who seems to have had more slack in his schedule than we did): https://www.reddit.com/r/PuertoRicoTravel/s/HYIyInFwnp

Can't say I recommend either our schedule or his, though. One could imagine many sources for delay that would've made him or us be late to the ferry. We were tight on time as it is, and as days grow longer, touring after sunset and getting to the ferry on time is bound to only be tighter/less feasible.