There actually are a trio of parks here, right in a row (from north to south):

The one farthest to the north (and the largest) is Recreation Park.  Right below (to the south of) it is Colorado Lagoon Park (where they filmed the 3rd Season scene of the police discovering Miguel Prado's body), and right below that (farthest to the southeast) is this, the smallest of the three, Marina Vista Park.

But it actually isn't a small park - it just looks like it compared to the other two parks.  Marina Vista Park has 18 acres, containing two soccer fields, tennis courts and a softball diamond.

(Note in the photo below that the outbuilding is no longer blue.)

A.  In the early seasons, I usually had to hunt down the locations after viewing the episodes, using clues from the various scenes.  By the now, though, I've developed a small group of fans, spies & tipsters who kept an eye out for Dexter filming in their neighborhoods, and let me know in advance when something is about to film there.

Between those reports, my own personal reconnaissance around town, and a few new resources I discovered, by the time the new first episode airs, I already know most (but not all) of the filming locations, and only need to watch the episodes and match up the scenes with the correct locations.

This group of helpful fans includes Rick, Ellen, Suzi, Elaine, Mia, Kerry, Joel, Eric, Jeff, & others. My thanks to all of them.

Rick tipped me to this one. And Julie also let me know that they were filming in the park. And I was able to verify the exact spot in the park because in the scene you can see a blue, park restroom building (with an even smaller blue box nearby), the only building in the park.  (Thanks, Rick & Julie!)