Its a long way to go for a day trip. I don't think there are any actual trips but you could get a coach or taxi. Its about 2 hours drive. You could stay over night like we did this year. We booked a room for one night at the Mave Belche Hotel (INVALID URL

You could catch the local bus from Marmaris to Fethiye( approx 2 hours) bus station. You would then have to go outside of the bus station to catch one of the many Dolmuses to Olu Deniz( approx 30 mins).

When you get to Fethiye bus station ask the driver for the Olu Deniz dolmus and he will point you in the right direction.

We are just back from Fethiye. We took bus to Marmaris and it took a full 3 hours as it stops at several bus stations en route. Buses are every ½hr. From Fethiye allow another 25mins to get to Olu Deniz.There are no boat trips, its too far. You can do the trip in one day, but it's a lot of travelling.

Rather than attempt to make our own way we did do an organised coach tour from Marmaris to Olu Deniz a few years ago.

We were told we would be picked up by coach from our apartment but the bus was quite late in coming for us (no surprise there really!) We then went round Marmaris to many other hotels picking up others before we started our journey. The trip included a stop somewhere for lunch and a swim (1 and a half hours). I do remember we had a bit of a trek, in all the heat, from leaving the coach to getting to the bit of the beach that we settled.

Truthfully we found it a bit disappointing as we had very little time to relax and spend at the Lagoon. Came back in the evening shattered! An overnight stay would be a good idea.