A day trip to Alibaug is entirely doable. The constraint is the ferry, and everything else follows from it.
The shape of the day
Early ferry out. The earlier you leave, the more day you have. This is the whole game.
Eat on arrival. You will not have eaten properly. Do not start the day hungry — we are two minutes from the jetty and open from 8:30am.
Pick one destination. One beach, or one fort. Not both. Travel time within Alibaug is the thing people underestimate.
Late lunch. Long and unhurried; this is the actual point of the trip.
Sundowner, then the boat. Check the last ferry and work backwards. Miss it and you are driving the long way round.
Realistic expectations
You will not "see Alibaug" in a day. You will see a bit of it slowly, which is better.
Where the all-day kitchen helps
Day trips run on odd timing — you eat at 11am or 4pm because that is when the gap is. Most places here close between services. We do not: 8:30am to 11pm, straight through.