Stock your boat with standard first-aid equipment: self-stick bandages, gauze pads, rubbing alcohol, OTC painkillers, antiseptics and other useful tools such as blankets, flares and flashlights.
Store enough life vests for everyone on board (and make sure you and your passengers know how to use them).
Tie down everything on your boat: life jackets that aren't in use, fishing rods, your cooler, etc. When you're out on rough water, items can come loose and cause damage to you, your passengers and your boat.
Passengers--especially children--should sit in full view of each other when the boat is in motion.
Use a weather radio so that you can keep track of weather changes.
Maintain your communication devices in working order. CB radio, cell or satellite phone are essential when must summon help, so check on your batteries and your power supply before casting off.