Far as I know, there are only two ports of (airline) entry in Costa Rica, one in San Jose and the other in Liberia, both many hours from the Osa peninsula. (Might be an airport in Limon, but not an international one?) Also, there are only a couple roads into that area. So the logistics and cost of having tools delivered to somewhere on the Osa Peninsula could be a problem. Your best bet might be to try to rent what you need in San Jose and rent a vehicle to drive down to Osa. If you can, get there early enough to take some time to check out some of the sights along the southern coast, e.g. Jaco, Manuel Antonio (best beach in C.R.), Domenical, Uvita, Ojochal, etc., on the way.
Ocean swimming is fine in Manuel Antonio National Park and around Jaco, but it can be cold and rough in other places. You also need to beware of large crocodiles, both fresh and salt water varieties, too. And, sad to say, there is a lot of petty (and not so petty) crime down there, so never leave anything unattended, not even for a minute. Corcovado Park is a popular destination for hikers and should be safe, although really remote. The amazing variety of birds and monkeys and sloths will make it worth the effort.
Good luck, and be sure to send us some pictures of your great adventure.