Re: 18ft bayrunner mods to make sea worthy
There isn't anything you can do to increase the seaworthiness of the hull without compromising it in some other way. It's made to be safe and comfortable within certain limits. If you want a more seaworthy boat, buy one instead of trying to engineer your own. That said, most any boat can go in any water when it's calm.
First, you need to use common sense. Stay close to shore, keep a close eye on forecasts and know enough to spot deteriorating conditions. Act quickly (head in) at the first hint that wind and waves are kicking up.
Second, you need to raise your level of preparedness. Always wear a PFD. File a float plan anytime you plan to head out. Put together a ditch bag and keep it close at hand. Additional battery(ies), bilge pump(s), GPS, VHF radio with DSC, connected to the GPS. A second, hand-held VHF radio. 'Upscale' items might include an 8-10 hp kicker motor and an EPIRB.
Others will have more ideas.
My .02