If you're doing a commercial tour for around $150 you're pretty much going to be with a cluster of 50 boats no matter what and time in the water is pretty much regulated. You can do the more private one for $1500 as posted above.
However, I'll tell you a secret, if you book with a Cancun outfit and go out on a Sat or Sun, good chances are your boat will have a portion of people who partied hard the night before (IE drunk with bad hangover), usually they get sea sick are sick as dogs by the time the boat finds the whale sharks, some won't even go in the water, a few will do the obligatory one time in and be done, more time in the water for you.