When you get to Hawaii, you can rent a snorkel set for really cheap and try it out really close to shore and see how you feel.
With the ocean, you don't have to do anything to float, so you should not be exerting any energy besides breathing through the snorkel.
Which island are you honymooning on??? We live on the Big Island. I would be more than happy to give you feedback on some of the touristy stuff <img src="i/expressions/face-icon-small-smile.gif" border="0">