I'd guess you didn't get the boat in the proper orientation to the wind. Likely the wind wave pressing down on the sail instead of helping to lift.
RIGHTING YOUR HOBIE
On that reach...flying through the air and water then...WHAM!!! Your Hobie Cat has forced you to swim instead of sail. Not really what you had in mind? if you have planned ahead you will be up and sailing again quickly. Whether sailing singlehanded or with a few friends, righting your boat can be quick and easy. First, preventing your mast from becoming one with the bottom, or turtling is a priority. We at Hobie have designed mast floats to help keep your Hobie Cat from becoming a mudhen. The floats attach to the top of your mast and will prevent the mast tip from sinking. The next objective will be to get back to sailing! Righting systems vary a bit but the theory is the same. A line is attached to your Hobie, thrown over the hull in the air and then weight (yours and or your friends) is used to lever the boat back to an upright and sailable position. After you have checked that everyone is OK, turn your hulls and mast into the wind (see diagram)
and uncleat your sails. You and your friend now need to congregate on the hull that is in the water. Grab the righting line, lean away from the trampoline and get as close to the water as you can. Try to stay out of the water. If you don't have enough body weight to right the cat we have the Righting Bucket! Just attach it to the boat with the hook that is provided, throw over the upper hull and fill the bucket with water! With the bucket strap over your shoulder, lean your weight out from the boat. The bag comes clear of the water and up the cat will come!! Grab the dolphin striker or crossbar as it comes up, climb aboard, sheet in and GO! Tipping your Hobie over can be a great way to cooloff and enjoy the water...try it! It's excellent practice for when you weren't planning to go for a swim but your Hobie decided that you needed to cool off!