Hi,
To get to Oludeniz your best bet would be to pick up a coach from Marmaris Bus Staion. They leave for Fethiye (which is where you need to get to for Oludeniz) about every hour. The journey takes 2.5 to 3 hours (which includes a short stop for refreshments) and is very cheap; off the top of my head I think it costs around £7.00 - but don't quote me on that.
From Fethiye Bus Station you would then pick up a dolmus for Oludeniz; it takes about 20 minutes and costs 2.5L
The coaches in Turkey are very reliable and extremely good value for money. They are well-maintained, comfortable and have air-conditioning.
If you wanted to take a taxi from Marmaris to Oludeniz - I'm hazarding a guess here - I would say it would cost somewhere in the region of £70.00 - perhaps as much as £100.00 - and you'donly save yourself about an hour's travelling time.
Water taxi's don't do that route I'm afraid; but if they did it would take about five days to get there!
Going by taxi would probably knock about