I Googled the term the other day and I swear you can find a blog or a website advocating sailing in bathtubs as the safest means to cross an ocean. One fellow who proclaims to know about seaworthy characteristics says to buy a Hinckley, the Rolls Royce of sailboats, because they have solid steel rigging instead of braided wire. I've never seen braided wire rigging on a boat. I think he means solid rod rigging versus 1x19 stranded wire rigging (19 wires wrapped in a spiral around one wire). He seems to equate rod rigging with strength in an offshore boat, yet you cannot coil up spare rod rigging and bring it aboard with you, but you can coil stranded wire as spares. You cannot splice a broken rod shroud, but you can do an emergency splice on stranded wire rigging that will save your rig and get you to shore. Hummm, maybe, just maybe that's why serious, seaworthy offshore boats have stranded wire rigging. Do ya think?
Another fellow actually argues on his site that lack of a keel and water ballast is the answer to a seaworthy boat--now that's a stretch and tall tale most sailors just can't make; but he does. He says if your boat has a keel and it runs aground, the boat will topple over. Therefore you should buy the kind of boat he has, an unnamed 20-something foot sailboat that can carry a 50 hp outboard on the stern. Yeah, right. Lots of those are making long passages!
An old friend of mine, a farmer from Minnesota, built his own boat, a Bruce Roberts design, from plans. He was used to building cabins in the woods, so he built his boat pretty much the same way--house wiring, PVC piping, and to be safer, the hull and deck three or four times the thickness specified by Mr. Roberts. His boat rode two feet lower than her DWL when launched. Seaworthy?
The interesting and sometimes amazing thing about sailboats is the diversity of their design and variety of opinions of their owners. Sailboat "captains" will come almost to blows over the stupidest things, like solid wire rigging versus stranded wire, or crimped wire connectors versus soldered wire, or CQR anchors versus Bruce anchors. And unlike religion or politics, there is but a narrow vale of patience for differing opinions before the captain declares you a total idiot for not agreeing with his view on the subject of keeled boats toppling over when aground and how wrong that is. I was almost thrown off a neighbor's boat once for disagreeing on the subject of crimped connectors--now there's something to ruin a friendship over!
I made friends with a fellow at a boatyard once who was rebuilding a CSY to sail around the world. He had done a great job cosmetically, but was mucking around with the rudder. His boat had a lot of weather helm--CSY's are very beamy boats with shallow modified keels and they will wear you out with a heavy helm. His solution was to make the rudder larger. It's a common solution and a wrong one, but you couldn't tell him that. I told him he just needed more headsail or more bowsprit or a smaller main--he would have none of it; the rudder would fix everything. Of course it didn't. All it did was act as a larger lever and decrease the force on the helm, but the rudder was cranked over just as much, slowing the boat. For someone bent on sailing around the world, you would think he would seek broad advice. Nope. He read a book or, more likely, talked to someone who also modified their rudder and so his course was set.
Speaking of rudders, my neighbor for a while at a marina in St Pete sailed a very pretty Catalina 30--another boat with a lot of weather helm. His solution was remove the rudder and install a factory fix, two rudders welded in an inverted Vee configuration. I told him all he would do is overstress the rudder post and the rudder post bearing. He would have none of it--he paid a yard a lot of money to install the expensive rudder(s) and in the first blow, bent his rudderpost and jammed his rudder(s). If you sail long enough, and spend enough hours in dock chat and boatyard chat with your neighbors you will hear it all. Like the owner of a beautiful new Sabre who polished (and ruined) the gelcoat of his topsides with Flitz metal polish.
Opinions of sailors seem often 90 percent justification for what they own, what they have built, what they have modified: they read a little and they become advocates for that author's often equally idiotic views. God protect small animals and sailors! In fact there is rarely any single answer, any single unlimited truth to seaworthiness. You can cross oceans in anything--in boats as small and unsafe as bathtubs or in the the best ocean-going sailboat--either may provide a safe arrival, or may provide you with tall tales, or may drown you in the process--the bathtub or the Hinckley. Expense is no guarantee of survival; Hinckleys have broached, suffered bulkhead failures and rig failures. They are lovely boats but they break just just like anything else. Size is no guarantee either. Lin and Larry Pardey have sailed around and around the world in small sailboats, 30 feet and much smaller.
In my view, seaworthiness is more a state of readiness and preparation than anything else. It is not the depth of the keel, but its strength and efficiency. It is not the solid or stranded wire of the rig, but the health of each component of the standing rig: the wire, the toggles, the turnbuckles, the mast tangs and the chainplates. It is not the displacement, but the usability of the boat: it can be sailed, managed and repaired by one person. It is not the number of crew, but their experience, their ability to adapt and improvise, and their desire to succeed. If you have these seaworthy attributes, you can sail that bathtub anywhere you want.