Yes, it is possible to book the return seaplane journey while on the resort. I think it is around $120 pp. However the return trip is less bumpy. Apparently this is usually the case.
The boat transfer was worse than I expected, as it just jolted you around so much. The weather was calm the day we travelled, but sea swell meant boat leapt out of the water and crashed back down again. My insides felt like they had been realigned. I personally did not feel sick. A tip is to sit forward in your seats with your back away from the seatback and hold your hand luggage, keeping arms and elbows well away from the sides.
There was nothing to see either as you are in open ocean. I would definitely do it again though if I were trying to keep costs down.
If you are young and able bodied you should be fine.