If you hire a speed boat to yourself it will cost you upwards of $250+ , You can easily reduce that to around $30 a head by joining a tour and travelling with other people. There are tour desks every where in Phuket just shop around when your there. They will pick you up and return you to your hotel, they also provide snorkelling gear and lunch on Phi Phi.
The only time to avoid masses of tourists at Maya Bay is probably at night time. Its always busy. The earlier you get there the less the crowd. Last time I went was very early morning and we were still queing for a mooring space.
Alternately you can catch the morning ferry and return late afternoon for about $5.
There are many more beautiful islands In Phang Nga Bay that are totally isolated and better for snorkelling etc. Ask them at the tour desk, I wouldn't bother with Maya Bay again.