Hi Adam,
My father-in-law has a house in Javea, so I know the area very well. So here's my potted guide to recreational freediving spots in the area:
Cabo de San Antonio.
This is the big promontory that separates Javea and Denia and has the marine reserve. Pretty good, especially right out at the tip, where it gets nice and deep very close to the cliff. It is a bit of a swim from 'civilization', but there are plenty of little caves along the way to keep you amused. The little cafe at Playa del Pope is perfect for a post-freedive beer.
The 'main drag' between Javea port and Arenal. Forget it - nothing.
Cala Blanca
At the other end of the bay from Javea Port, beyond Arenal. OK. Not that deep and requires a fair surface swim to get to the interesting bits. The section towards Cala Sardinera seems quite popular with the local spearos.
I discoverd a nice swim-though at about 10m that was home to a friendly octopus.
Playa del Portixol
If you like exploring caves, this is the place. There are a couple of really big ones on the south side of the bay - big enough to get a small boat in. Best if you explore these early in the day, before the jet-ski tours show up. Take a torch.
Around Cabo de la Nao
Never freedived around here, because you need a boat - it's too far/too exposed to contemplate a surface swim. However I have scuba-dived and reckon it would be great for freediving. in particular the channel between Ambolo and Isla del Descubridor. It's on my 'must try' list.
Granadella
My favourite place. A little narrow bay, very pretty. I like it not because it's deep (don't think I've managed to find anywhere much beyond 15m) but because it's full of little caves, swim-throughs, canyons, etc. You can spend ages just bimbling about and exploring. Ignore the sandy bottom in the middle of the bay (unless you want to wave hello to the scuba trainees practising their mask removal). Follow the edge of the bay on the left or right hand side. A popular spot with the local spearos. Nice little restaurant for lunch.
Cala Los Tiestos
Finally, something a bit 'different' This is a small bay you get to by driving through the Cumbre del Sol "urbanisacion". Enter the sea by the car park and swim 200m or so to the right. There is a section where the cliff has eroded away a good 5m back from the face just under the surface. You can remove you fins and weightbelt and stand upside down and 'walk' along the underhang. Pretty cool.
Have a great time - it's a lovely part of the world.
Ian.