Definitely get there early. A good hour or more before the boat is due to go if you want to catch the first boat at 10am. Last week I got to the landing stage at 8.20 and was about 6th in the queue (I was camping in Marloes, 2 miles down the road). By 8.45 there were at least 50 in the queue.
Also be aware that the local Wildlife Trust organise guided walks on a Wednesday. They have priority on the first boat of the day, even if they turn up after a long queue has formed. They then jump the queue to the front which causes some disquiet amongst those who got there early and don't make the first boat. However, if a very big queue has formed the first boat may well depart early (9.30am). The boat takes 50 people at a time.
It's £9 for the return boat trip (they prefer it if you have the right change), and £6 landing fee for the island. The car park at Martin's Haven is £3 for the day.