Hotel guests get pool access on a "first come first served" basis, meaning basically: show up really early in the morning if you want to avoid the hoards of hipsters that will likely be crashing your party later in the afternoon and evening.
Meanwhile, non-guests can log onto the hotel and pay $45 for a day pass on Fri-Sun, or $35 during the rest of the week. The fee includes the possibility of snagging a chaise lounge or bleacher lounge (depending on availability, of course), plus towel service and access to the hotel's WiFi. Additionally, "Frequent Swim" packs are sold at discount prices, offering 10 passes for $250 and 20 for $400.
It's important to note that tickets are only sold up to 24 hours in advance, so you can't really go on and book a pool date for the following week, however much you might want to.
For all your other watery needs, you'll be wanting the poop-filled
cesspool over across the street in McCarren Park.