I would actually say that toys with an open DC port are often not waterproof, since water getting into there is often bad. There are exceptions, like many of LELO's toys, but generally I've seen full waterproof toys using magnetic charging.
Depending on the manufacturer and the warranty, if you're still unsure after what Stormy said, you can always try to use the toy in the water. Be very careful and be sure to read through all of the papers that came with the toy before doing this though!
You could always contact the company directly as well, or read through the reviews of the toy!