When booking our excursions on Navigator of the Seas, we noticed that the beach trips said to bring a towel. Will these be supplied on the ship to take to the beach or do we need to actually pack beach towels to bring with us from home?

Just take one with you from the ship. Typically the ship provides towels at three main points; your stateroom (bath size towel), the gym (small handwash type towel), and the sauna/locker room (wraparound size).

Are they provided at the pool as well?

I've been to ships that sell beach towels (nice ones) by the pool side and others that don't even have them available nearby. On this one, there isn't really one answer, if you have a favorite beach towel, bring it, but I honestly think that is kind of nutty, just use one from the ship.

We used to bring ours but learned that was an unnecessary waist of suitcase space. We get beach towels from the pool and take those. On Carnival you must check towels out then bring them back or you're charged for them. Most ships also have beach towels, signed out, as you are leaving the ship.

Yes, Royal Caribbean supplies beach towels that you can take ashore with you. Just remember to bring them back.