$6 or $8 is basically what tanks and weights cost anywhere. Wouldn't be a factor for me. As to where to dive, I'd say based on a Park Royal stay, you might want to check out Scuba Shack. It's bright yellow and on the south part of the Atlantis Submarine building. (I think) I stay at Casa del Mar, and the last time I was there, Jan/Feb, we had a hard time getting tanks. I think they're compressor was broken or something. Never had a problem at Scuba Shack. They have a pretty easy water entry into a protected cove where their boats load, but watch the waves when you get in. They can push you around and into the rocks. From there, we go north, away from the cruise ships and toward Casa del Mar. You're generally swimming with the current out, and against the current back, so that's not optimal, but doable. Park Royal, Scuba Shack, El Cid, Casa del Mar, anywhere there watch the boat traffic and you must use a dive flag. When I was there last Jan/Feb, there were seahorses in front of Casa del Mar, or around that area anyhow. I saw/photo'd two, and saw pictures of 7 different seahorses. Don't know if they're still there now. It sprung a new business as there was at least one freelance DM who was leading people on dives there to see the seahorses.
I actually dove with Scuba Shack for a week a few years ago, and they were great. Very accommodating. Same experience renting tanks from them. If you don't have a dive op lined up, you might want to consider Scuba Shack. They have very convenient facility, it's easy to get on their boats in that cove, and they have lockers where you can store your gear. Basically, all you have to do is walk across the street from the PR. The shop owner, Roberta, is very nice as are the rest of her cast and crew. Oh, and she was putting this amazing, like, bundt cake on her boats for the SI. Sooooo good. I've been diving with Deep Blue for years, but if I wasn't, I'd probably dive with Scuba Shack. Extremely convenient from Casa del Mar or Park Royal. (Or from Village Tank Haa, Roberta's "hotel" right across the street from Scuba Shack.)
Casa del Mar doesn't charge for boat pick up, but $5 for a week sounds very reasonable. I think Mossman, on this board, stays at PR and would have better info on staying there than I do.
Casa del Mar has WIFI in their lobby. I don't know if they charge or not, but cruise ship workers used to come there to get on the net. If you stay at CDM, there is no charge for internet unless you get the premium service which is available in your room. I was able to connect on the free service in my room so I didn't bother with the premium.
Don't know where to exchange pesos around there. Maybe Mossman knows. We always do that at one of the cambios near the square. Even if you don't get the very best rate, it's still a money saver.
Hope this helps,
-Blair