Buch of my friends are planing on a half day trip should we book in advance? Or just get there early to make sure we have a spot on the boat

It depends, 4 or 5 isn't too many but if you have more it might be better to call. You might get a group rate.

You can usually get a spot as long as you get there early

Book in advance, its just easier knowing you have a spot when you get there