From what I could find there doesn't seem to be much in terms of organized tours to Playa Larga but the Bocas del Toro site mentions that you can get there by plane or driving. From another site there is a mention of:
Long Beach, on the north of Bastimentos Island is a large beach with a very rugged beauty. There is where marine turtles come to nest during the monthes of april to october. It is difficult to get to due to the rough seas.
That is possibly the reason. There is a site, however, that mentions the village of Salt Creek from which you can get a land based tours to Playa Larga but it doesn't seem to have any online booking presence, which may mean that you have to get it while you are there.
Considering the description of the village I found on Wikipedia. The lack of web presence doesn't surprise me one bit now. Found the link with a contact information for Salt Creek Tour Alliance which may be worth a look and another Bocas del Toro tours, which also may provide turtle watching.