How can creatures fight in cramped spaces like on a boat?

Question

The rowboat in the core rules is stated to weigh 100 pounds. It is further described in Ghosts of Saltmarsh to be large enough for 4 creatures (2 crew, 2 passengers), and sized 10 feet by 5 feet. This is similar to this question about flying carpets, but here the rules explicitly tell us that 4 creatures fit into the boat, so there is no question about how many creatures actually can use this vehicle.

This however leads to a similar dilemma of how to deal with creatures that are densely packed at the beginning of combat, and under circumstances that make it undesirable or harmful for them to change that arrangement, and maybe this here is a better example, involving only mundane objects: what happens if the boat now is attacked?

Say, traveling through a watery swamp by boat, the adventurers are attacked from the water by Lizardfolk. It seems ridiculous that two of the four characters would jump into the water, because they "cannot willingly end their turn in another creature's space". In a similar combat encounters I have experienced as a player, the DM handwaved the space requirements for combat, allowing the PCs to fight back from the boat.

It feels even more ridiculous with a longship (70 ft. by 20 ft., 140 passengers and crew), which could accommodate just 56 medium or small sized creatures under combat conditions. So, every time two longships engage in battle on water, most of the warriors on board have to jump into the water? That is surely not how real-world sea battles have played out.

Are there rules that can guide handling an exception here, in addition to rule 0 (for example, fighting with disadvantage due to cramped circumstances)?

Answer 1

RAW doesn't explicitly cover your circumstance, but let's insert some practical DM logic with the rules that do exist.

Let's start with the problem rule- you can't willingly end your turn in another creature's space, DURING COMBAT.

That during combat part is important- we'll get to why in a second. The next thing we want to do is look at "squeezing" rules.

Squeezing into a Smaller Space: A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space.

Emphasis mine, we now have a conflux of conditions that can be applied to your unusual situation.

1: The restriction on sharing squares occurs strictly during combat. No one in their right minds is going to debate that you can take two average people and have them huddle up, side to side or back to back, in the same 5x5 square. However, taking two people who are attempting to fight for their lives and having them stand in each others' space like that isn't very sensible.

2: We're going to skip the rowboat b/c a cursory glance at the picture on D&DBeyond for it seems to just be a sideways longship in its gridded dimensions. Let's look at the actual longship, instead. It has sixteen rowing benches, each of which occupy a single 5 foot square. You can reasonably seat two people on these benches, which accounts for 32/40 crew members, and you can then start stuffing passengers into spaces as thick as you can fit them.

If we combine one and two, we come to the reasonable conclusion that regular passengers who aren't fighting don't need to be made part of the initiative- they aren't in combat, so they can share space with other people who aren't in combat. People who are fighting will have to stay out of their space but unless you fill the boat to capacity you should still have combatants capable of moving around the ship, although probably treating much of it as rough terrain.

Optionally, if you wanted to insist on stuffing the boat to the brim and still trying to fight, you could institute the squeezing rules for characters sharing a space, to represent the difficulty of trying to attack, defend, and not trip over each other in such tight quarters - Disadvantage on attack rolls, dex saving throws, and attacks against it have advantage...personally, I'd jump in the water voluntarily to avoid being a nice juicy AoE target with disadvantage on Dex in this case, though.

One last note- it's worth noting that the longship has a 20 foot mast with sails and rigging. This means there is also space that can be occupied beyond what you see on the surface of the longship.

Now we can get back to your rowboat example; taking the above logic, you could squeeze 4 creatures into a 10x5 rowboat... but it probably wouldn't be terribly safe or efficient for all of them to be standing up and trying to fight at once.

Answer 2

I'm not aware of any specific rules. A bit of a frame challenge though: not all party members can take part in active combat in a boat anyway. Rowboats are quite unstable. Walking in one is heavily discouraged, as capsizing a small boat can happen easily. Add some adventurers loaded with armor and start swinging swords and warhammers around. I'd hazard half of the crew is needed to try and balance the boat to avoid capsizing. Swimming enemies might also try to tilt the boat, adding for the need for balancing the vessel.

Trying to reach into waterline for stingy-stinging might need another a party member to hold one by belt to keep your buddy from falling over. Should you do a dip, at least there is no heavy penalties for swimming whilst wearing armor. Please try to not flip the whole boat over, as all of your equipment, including backpacks, would be sinking like stones. I wonder where the wizard did stash their spellbook...