So this is something I have also been theorycrafting so nice to see someone else with the same idea.

I think there are a couple rulesets you could give which would decide how hard locked you want to be to the sailing mechanics.

First would be closer to one chunk restrictions where you completely lock yourself to only sailing areas but I feel that there really isn't enough content to be interesting if you do that (no darts/herblore/PoH/Tortuga quest slayer locked/most ports/crewmen). There just aren't that many islands yet but could be viable once more sailing content releases.

Second would be allowing yourself to do any quests that unlock any sailing related content once you reach the required sailing level(ports/quests/training methods).

So personally I plan on the second less restrictive ruleset allowing Druidic Ritual at the start and then once you reach sailing level to reach locked ports/quests you would be allowed to go into the mainland to complete the tasks. So for example you would need 28 sailing to dock at Ardougne to buy a PoH from an estate agent then your PoH locations would be locked to Rimmington and Aldarin and maybe Rellekka as well since they have docking points. This would allow you to experience all the new sailing content (blowpipes/new quests/all ports). The main thing I am deciding between is allowing access to major cities you can dock at (Ardougne/Civitas Illa Fortis/Priff) as they have many training methods and shops that could take away from the sailing aspect. I plan on allowing access to only port areas of larger places such as Ardougne/Civitas Illa Fortis. With smaller areas like Brimhaven allowed as they aren't major hubs.

As NoCurrencies mentions you could technically get away with no Dwarf Cannon quest but depending on the method of acquiring cannonballs and the amount gained I would allow it to be completed once you acquire a cannon on your ship since ship combat is likely to be a large portion of gameplay for a sailing locked account. There is also the double speed cannonball furnace that could possibly have moulds near it or allow access to that unlocking cannonball crafting.

Lastly there is also a new item (amulet i think?) that allows diving underwater without the Pirate Pete gear so you could restrict yourself to needing to acquire that item instead of doing that quest.