Yes, it will have sold out, 2 or 3 months in advance, plus you'd end up getting an afternoon swim time.
Showers usually happen in the afternoon. The mornings are sunny. This doesn't really matter because the swim times are done by 2 or 3pm, but most people want the early time because they are so excited.
If you go on the
www.discoverycove.com web site, register, and start the booking process, the computer will let you see a color coded calendar showing which days are available. You can back out at this point without booking anything, then come back at a later time and check again. Don't worry, they won't spam you with e-mails after you sign up.
Start looking at the calendar and you'll get an idea of how fast the days are selling out. It's hard to judge right now because it's winter and only Sundays and Thursdays are gone. But as we move towards the summer months, you'll see days sell out.
I wouldn't worry about the weather that much, you can't control it.