I have decided to come back and justify a lot of my suggested additions, because this seems unpopular, likely because I framed it as just "Nerfing Boarding"
Of course, these ideas would all add complexity to the game, because I'm actually a game designer myself, and these weren't just spur-of-the moment ideas.
First is that being boarded without being good enough to fight them off feels awful to fight, as you just spawn die, and while you'd likely lose the naval fight anyways, you'd feel better about it, as you'd do damage to their ship too. If a game feels bad to play as an unskilled player, it should work to offset that, so new players feel good about playing, especially in a game with as sparse opportunities to practice or learn as Sea of Thieves.
Number two is that these extra options to avoid boarding create many new strategies and archetypes for combat, which always makes a game better.
If you are good at both Boarding and Naval, you'll want to get close to harpoon aboard, firebomb them, and get away to attack their mast while they worry about the fire.
Bad at naval but good at boarding? Anchorballs, ammo crate, harpoon, firing yourself from the cannon, and the cutlass are all going to be a staple of your strategy
Good at naval but bad at boarding? Anchor lock, roll up the ladders, and visit small islands to find splinterballs and bear traps
Bad, generally? See the above section, but increase the range a few fold
Are you wanting to board but you're a solo-crew? Do you roll up the ladder to keep your ship safe while you wreck shop, or leave it down for you to escape?
Do you often use anchor-drifting to maintain a good shot on fast ships? Do you lock the anchor to avoid boarders dropping it at the wrong time, or do you keep it open for you to quickly adjust angle?
How would the popular anchor key spots develop over time?
A new, powerful, but rare ball would give reason to visit small islands for supplies; a new type of bigger, more damaged hole would leave opportunities to buff or expand the sea creatures, and maybe make megalodons an actual threat; having to hide items on your ship could lead to a reason to add mechanics for that, which could then possibly be expanded into a new loot system, like carrying messages between outposts, or some kind of item that accumulates value the more activity it sees on the seas, or some other treasure with incentive to hide it away on your ship.