Hello,
400 lbs is alot for a beach cat. Get one with high volume hulls like many of the 18-20 footers mentioned.
If you can find one a Gcat 5.7 can cary loads of weight and is simple and fast. we had 1 in the rental with a short mast, and i was always amazed at how fast it was with 6 people on board. still had lots of freeboard w/6 up as well.