Whether you need a visa for Turkey or not depends on what your nationality is. (Trip Advisor profile location doesn't necessarily indicate nationality). There are also visa exemptions for some nationalities who would otherwise need one, if they both arrive and depart by sea, and stay in the country no longer than 48 hours. They're intended for passengers on cruise ships, but also work on day trips like Rhodes-Marmaris-Rhodes.
One caution if the OP does take the day trip. Some of the packaged excursions throw in a "free" guided tour of Marmaris. This seems to consist of being taken by coach to a carpet shop and a gold showroom, where sales staff try a hard sell on you. Even if you're in the market for carpets or jewellery you may find better value in the main shopping district.
In addition the visa website linked to in post #2 is not the official Turkish Government one, and is a private moneymaking enterprise that charges more than the official site https://www.evisa.gov.tr/en/info/
Thanks for the Visa info Andy, much appreciated
how long is the boat trip
Hi John R. Depending on which boat you take either just over or just under an hour. Enjoy.
how did you find the trip, worthwhile ?