Hi there, you will find plenty of boat trips and it will be cheaper to book direct with the boat captain and not through an agent.
Olu Deniz is quite a long trip - you catch a bus from Marmaris bus station and then another bus from Fethiye. I have done it but can't remember how long it took, probably about 3 hours. Buses are OK and frequent and very reasonably priced. You will find travel agents doing trips there but it will be more expensive. A nice area to visit is the Datca peninsula and it is just over an hour over spectacular mountains. The buses go from the bus station more or less hourly and come back more or less hourly. It's a beautiful trip to a place that is very different to Marmaris. There are nice beaches and a quaint harbour. For beaches around Marmaris, I think Icmeler and Turunc have the best ones and you can visit by dolmus or boat.

Beaches in the area are fairly poor. Marmaris is very stoney/shingle, some parts of Icmeler beach are OK not a great deal of sand and again quite stoney/shingle. There are plenty of sunbeds and fairly cheap, £1 per day. The waters are fairly calm and safe. From memeory Turunc beach is very similar. Turunc is a good day out however. Take the dolmus to Turunc, the views over the mountain road are spectacular. Once there enjoy the day in a laid back palce before taking the water taxi back to Marmaris

Stewart