score:3
I've not done it, I wouldn't have thought you'd have needed to book for in advance. I certainly hadn't heard of that, but Wikivoyage says:
If you decide to go solo, you can only buy tickets for the Underground River tour in their office in Puerto Princessa as that is the only place to get a permit. There is a limited amount of people allowed each day and booking office is more interested in selling tickets to tour agencies, so make your reservation at least 2-3 days in advance but it can be booked out a month in advanced during holiday seasons. Permits are no longer available in Sabang. An online booking system is in the works but currently unavailable at time of writing (May 2012).
Which makes it sound like the tour agencies buy the tickets, but perhaps don't sell them.
Honestly, I'd be stunned if there wasn't a way to pick up a permit on the day (perhaps for more money) I'm sure the hotels probably reserve enough for their guests but if you want to be absolutely sure book in advance.
Also, April's not exactly high season over here (unless you're here on Easter weekend) so it should be quiet.
Upvote:0
Unless you have your own transport do take an organised tour. You may well be, like me, a fan of independent travel, but the "tour" is little more than a van from the city centre to the river with a couple of photo opportunity stops on the way, admission to the river, and all the assistance you might need. There's no great advantage to doing it independently. Incidentally, PP might "considered as the largest city in the country" but most of it is countryside and the river is in fact a fair distance away from where you'll likely be staying.
As for whether you need to book, that's a question which setting up the answerer up to fail. If I say no, and you can't get a ticket, you're going to be unhappy. Unless there is a PP travel agent on here you won't get a definitive answer because "it depends".
All I can say is, I didn't book until I got there and had no problem. The country isn't heavily touristed but do bear in mind that the river is a very popular spot with the tourists who do make it to the country and also with Filipinos.
My conclusion would be, book something when you have your dates finalised but don't sweat on it until then.
POSTSCRIPT: I just noticed that this is a rather old question. Perhaps you could feedback with whether you booked or not and if not if you got into the attraction ok?