Hi there, Molly!
Currently, the water Park pricing shakes out as follows (please note that these prices are subject to change at any time):
Blizzard Beach
$8.00 rental per day for Small: 11"H x 9"W x 16"D plus a refundable $5.00 key deposit
$10.00 rental per day for Large: 17"H x 9"W x 16"D plus a $5.00 key deposit
Typhoon Lagoon
$8.00 rental per day for Small: 12"H x 12" W x 16"D plus a refundable $5.00 key deposit
$10.00 rental per day for Large: 18"H x 18"W x 16"D plus a refundable $5.00 key deposit
While you can certainly rent towels at both water Parks, Guests are welcome to bring their own towels. You can ask the concierge at your home Resort if it is possible to bring some towels to the water Park; normally, towels remain on property, but if you explain your situation, the concierge might be willing to work with you. This is, of course, entirely at the Cast Member's discretion.
Have a great time!
See you later, celebrator,
-Deborah B.