you can have chances to have sun bath for free in Negombo
there are lots of facilities for tourists by hotels or you have freedom to be there as you wish (no in nude in public please - may be you can be in nude inside the hotel area)
there are lots of nice fishemans offer boat trips in the sea for tourists.
you can talk to some one when they are in the beach with their boats or you can ask from any one.
first ask for the price and time. they will give you a good price. beware from touts. you can bargain directly from the owner.
some time boat rider need one asiistant onboard. its better for you or may be you can ask for helper for rider