On the Spot booking is best option as it gives flexibility to choose from a range of activities and also gives you chance of saving some bucks by negotiation.
This may be the right time for the water sports in GOA, as there is too much public there and the best deal in found by negotiating on spot.
I got a deal in 2k for 5 sports and it's totally over your talk at the place.
Start by atleast 40% down their rates!