You do not need to be a strong swimmer to enjoy a Komodo tour. A large part of the experience happens out of the water: Padar viewpoints, beaches, Komodo or Rinca ranger walks, sunsets, island scenery, and simply living aboard the boat. Non-swimmers regularly take these trips and choose which water activities make sense for them.
If you want to try snorkeling, tell the crew clearly that you cannot swim before entering. Use a properly fitted life jacket, stay beside a guide, and start in calm, shallow water where you can get back to the beach easily. Some operators also carry life rings or other flotation devices that a guide can hold while towing a nervous guest slowly over the reef. That can be a far better introduction than being dropped into deep water from the side of the main boat.
You should skip strong-current locations unless the crew has a very controlled way to support non-swimmers. Manta Point, for example, can be completely inappropriate on a fast-current day. There is nothing unusual about remaining on the boat while others enter. A good guide will help you find the safe parts of the itinerary rather than making you feel that you have to participate in every swim.

