The beaches of the Marine Park vary in size and number of visitors, so do your research and pick accordingly.
I'm giving the park a "poor" rating simple because of the highly irregular and informal way of managing the boat system that brings you to and from the islands. When you buy your ticket you are asked what time you want to return. You're told to be at the jetty 15 minutes before your specified time to make sure that you don't miss the boat. However you are given no way to recognize which boat will be picking you up, nor are the boats generally color coordinated or have their logos prominently displayed. The effect of this is that once you are back at the jetty waiting for your boat to return it becomes an annoying game of "is this my boat?" wherein one must board each boat and ask the boatman whether or not this is the right boat.
Our first trip to Manukan island was without mishap, as a boatman magically knew which boat we were to ride on and guided us there. That was great but I'd seriously much prefer to know in advance so that I'm not on edge until I'm finally in a boat.
The second trip to Sapi island was a bit of a disaster. Our boat was to arrive at 1115. Being the paranoid sort, we decided to start hanging out at the jetty by 1045. Boats came and went, and with each departure we started getting more and more worried. When it was around 1120 and already 3 boats bearing our boat company's name had turned us away telling us to "wait 5 minutes", I lost my cool and called their emergency number.
Initially the man on the phone casually told me that we had "missed" our boat and needed to wait until 1215. This ticked me off, since we had picked 1115 specifically because our hotel check out was 1pm and we wanted to have enough time to take a proper shower before leaving. There's nothing more annoying than trying to be responsible for your time and then being told that it's your fault. After explaining that 3 of their boats had turned us away they replied that they would send over a boat to ferry us to KK immediately.
Turns out that the boat that was to pick us up had some sort of engine trouble, and we had slipped through the cracks. While I'm willing to accept that this kind of thing happens, it's unacceptable that we had to wait for 45 minutes on the jetty and had to have a freak out before it was resolved. If the boat companies had a representative at each jetty to coordinate and inform passengers about the status of their boats that would be great.
It seems I'm not the first person to mention this issue with the boats, so I hope this will be fixed soon. It almost ruined what was otherwise an idyllic vacation.