Part of the reason things have been quiet here lately is that I was taking a course on navigating vessels under sail in coastal waters, which pretty well ate my weeknights. At the beginning I was looking at doing a "Notes from a $TOPIC course" series like I've done previously with some courses [1][2] but it got away from me. So, having arrived, now reflecting.
One conclusion from spending a lot of time staring at charts and plotting courses with pencil, compass, and protractor is that there is definitely room for procedure and gameplay in modeling this.
I think the distinction between coastal and open-water navigation is probably under-appreciated in Trad D&D's "waterborne adventuring" rules. I don't know much about long-distance open-water passagemaking yet but reading accounts from eg Slocum it doesn't seem crazy to abstract down to "n days pass, have a couple random encounter rolls, you arrive more-or-less where you were planning to" given reasonably-intelligent play like using the trade winds and celestial navigation. You can definitely get screwed and capsized by a big storm in the open sea, but there aren't many things to run into. The coast, on the other hand, is by definition something you can run into. To paraphrase one old salt, "the sea is pretty safe; it's the land that will get you". Coastal vs offshore is probably also a useful line for ACKS' rule about continuing to sail through the night with a navigator - at night in open sea, by all means, get out your sextant, take some bearings on some stars, and sail on. At night in unfamiliar, medieval coastal waters without lighted buoys, an accurate chart, or a full moon... probably wiser to drop anchor and wait for dawn.
On a 1:80000 scale coastal chart like Chart 1210TR, one inch is about one nautical mile (some nuances apply due to Mercator projection but good enough for gaming work). A small vessel under sail typically makes about 6 knots. So if you do 1-inch, 1-nautical-mi hexes, you get a 10-minute turn to move one hex. Or you go up to one-hour turns with 6 hexes per turn. Either way, you also probably want to apply current smeared across the hour; if sailing in a 3kt current, you get pushed one hex current-ward every 20 minutes. Since we're stealing a real-world chart, we can go dig up published tidal current tables.
Putting it on hexes is also kinda reasonable for dealing with wind direction and tacking; pick (or roll) one of the six hex directions for the wind to be coming from, and moving directly in that direction costs double (assuming you're making multiple short tacks within a single hex). A modern sloop-rigged boat usually wants to sail at least 45 degrees off the wind, and then there's some leeway so you're going to do a bit worse than 45, often more like 50-55 in significant winds, sometimes much worse in big winds. 60 degrees off the wind is probably quite optimistic for medieval sailing vessels but if you're running a post-apocalyptic setting where not everything has been forgotten, primitive sloops sailing 60 degrees off the wind seems workable and simple. Roll again every hour to see if the wind has shifted direction or remains from the same direction (or if it dies entirely, leaving them at the mercy of current - which is how many boats without motors end up on the rocks).
The other side of tide from current is depth. A 1-mile hex probably admits multiple depth soundings. If the party is sailing in a hex where there are spots with a depth less than their draft, there's a chance they run aground. This seems basically like the "roll a d6 to see if the trap fires". Use your judgement of what fraction of the hex is very shallow to determine the probability they end up in it.
If shoals are traps, tidal straits (also called tidal gates) are doors. These are narrow channels through which tidal currents are very strong. Examples include the Golden Gate in San Francisco, Hell Gate in New York, and Deception Pass in Puget Sound. If you try to pass the wrong way through one of these at the wrong time, currents can be more than four knots and you make very little progress. So they're like doors that only open at certain times.
Finally, how do players actually navigate a coastal sailbox? They don't have a chart, and there are probably not lighted buoys in your setting. There might be lighthouses, and if they have a hand-compass you can give them a rough bearing. The various tall points of reference typically on coastal charts, like radio towers, water towers, and church steeples could be readily translated into towering keeps, wizards' towers, and hilltop monasteries for your players to go Full Lindesfarne on. Chart 1210TR includes "monument" and "Chilmark Spire" points of reference - I'm not exactly sure what the Chilmark Spire actually is but it's rather evocative, no? Again, in a post-apocalyptic setting, city areas could be readily translated into ruins. Headlands, steep coastal bluffs, and the edges of particular islands also seem like commonly-used points of reference for inshore work. We have, in the language and reference points of actual navigators, pieces for a language of wilderness play.
One may readily ask - why bother / will there be interesting gameplay here? I think so. Looking at the structure of river deltas, barrier islands, and sounds, there are lots of tiny islands and the structure of the water-network comes highly-Jayquayed by default. A small island is kind of like a room; it is self-contained and you can stock it as a unit and the party can anchor the boat and row ashore and explore it and interact with the stocked thing. And then your waterways are sort of like treacherous hallways, and your sheltered bays are places to rest at anchor.
In conclusion, perhaps deltas and barrier island chains may be thought of and played as dungeons, open sea as wilderness.
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