As far as I know, there is only one boat trip you can do from Plakias, and that is to the beach at Preveli. This beach appears on a lot of postcards. It is at the foot of a gorge with a small river running down it. The beach is a sand spit across the mouth of the river, so the river is at one side and the sea at the other. These days it is crowded with umbrellas, but I have a photograph of it when it was empty - see the link. It used to have a 'far end' as well, but now it's much too crowded. However, it's worth going to walk up the gorge amongst the palm trees and to enjoy the scenery. There are some rock pools you can have a dip in as well if you don't mind the icy water.
My WebpageThere are a few beaches within walking distance from Plakias but beyond that there is only Preveli that is within a boat journey and there are no other small islands to go to, so I think that conspires against there being a lot of boat trips. I have never seen anyone hiring boats either.
Greecemad.