They sail about 6pm and cost 150 rps, we went on Paradise which was okay

The 6pm cruise is the best to see the sunset
annx

Princess de Goa (at the far end of the jetty) leaves at 6.00pm, it used to be the Royal but that now goes about 10 mins earlier.
Cost is 150rps per person and a further 100rps to sit on the top deck, which has waiter service for drinks and snacks etc.. (worth the extra + better views)
You get your ticket at the kiosk, and pay the extra 100rps at the bar area.
This year was the only time we didn't get the sunset due to a cloudy evening (Boooooo !!) but another magic hour up the Mandovi we had.
Spike.

Many thanks for all the sensible replies.....