In 1974 I knew of a guy who ordered an H25 from the factory with two distinct modifications: he wanted the bulkheads painted white (Herreshoff style), not the teak veneer; and he had an inboard diesel installed by the factory. He meant to take it off shore, possibly round the world. I know the boat was delivered; I never heard about his adventure. This is the only H25 of that series I know of that had a factory-installed diesel inboard.
Aside from taking care to beef up typically vulnerable structures (rebed toerail, rebuild compression post, replace all hardware backing blocks, rebed keel-to-hull seam, etc.) that should always be inspected prior to depending on any boat for any length of time away from 'home', there are only two Achilles heels to the H25 series-1 boats that demand improvement. These are:
1. Cabinside windows (actually 'deadlights'). These are too large and fastened to too thin of a structure to be seaworthy away from a reasonable TowBoatUS call from land. The backing rings need to be replaced with something wider in area and better-bonded to the deck/cabin skin. I would strongly advocate structural-fiberglass board. While you are at it, consider making this backing board to include a central mullion down the middle (making the window appear as two-- or more-- from the inside. You can still cover it with one sheet of 3/8" black Lexan over the outside, which is the other required improvement to this element. No 1/4" Plexiglass window should ever go to sea.
I did not do this on Diana merely because I have become lazy (and broke); but it'll remain an area of worry till I do it.
2. Companionway threshold. Steve Dashew says that the cockpit should be remain seaworthy when completely full of water-- that means no potential water intrusion from seat lockers, hardware fasteners, gauges, and especially the companionway. I constructed a proper bridge deck across the companionway and filled the space beneath it, making a new, shorter companionway opening. This provided me with a mounting base for the traveler, a pleasant seat for conning the boat in poor weather, a safer companionway opening and a place (under it) for the day cooler and cockpit bucket (if you've singlehanded in a busy channel for any length of time you know what you use the bucket for). I would not recommend taking any boat with a deep companionway opening to sea. Forget the 'convenience' of being able to step through to the ladder. What is this, a formal dance? --there's nothing convenient about taking a small boat into the open ocean. A better mentality would be to compare it to taking a homemade spaceship to the moon; and I submit anyone looking upon the enterprise as anything less casual deserves the disasters he courts.
That said, the H25 is by design an eminently seakindly boat, what I call a 'hellandback' boat because that's where it'll take you. It sails well to weather, handles gnarly conditions reliably, provides a good turn of speed and is reasonably comfortable in all conditions. If you're determined to sail open water in a small boat, the H25 --given the necessary preparations and improvements-- is far from the worst choice.
The comment about carrying supplies is a good one. I provided so much 'extra' storage space on Diana that I can probably sink the boat 2 inches with fresh water and canned goods! (and not even clutter the countertops and bunk footwells) 'You can sail forever if you have food and water' = true. Remember that so long as the keel isn't bumping the bottom, the boat doesn't know how deep the water is. She will sail in 6 ft as happily as in 6000 ft.