In Manuel Antonio, there is usually a guy by the main entrance that rents snorkel equipment but he wasn't there the last time I went. May think of bringing your own as MA is a good place to do that with the sheltered bays. Probably as cheap in the States at Wal-Mart as it would be to rent them here.
Don't think you can jet ski within the confines of the MA park itself but have seen people with Jet Ski's for rent on the large, public beach just outside MA but not sure that would be a good place to do it with the waves/rip tides there. For that, think more about Jaco and the beach inside Los Suenos. Here is a link to one place that rents them and another to the Jaco area 'official' website...
http://www.allstarjetskirentals.com/http://www.jacobeach.co.cr/Never heard of the place you mentioned, but looks like a good tour they offer...