They can sink if they're damaged. I'm guessing the ships you're trying to sink are older ones that are still using hatches to let water in. Thanks to the pressure mechanics that were implemented in the space update, hatches let water in very slowly. A subsequent update to make hatches work better for that purpose... failed hard.

To work in the game is it is today, a sinking system needs to use explosives to damage the hull, so they need the weapons dlc too.

You do not need explosives.

The pressure was added as a major update to the base game. It was coincidental with the space DLC. It is here to stay and should be.

Sinking ships using hatches, sinks every time. https://youtu.be/ge4-z4YQ5O0

Sinking ships with door frames. https://youtu.be/4YMpgYviqb8

Originally posted by Captain:

You do not need explosives.

The pressure was added as a major update to the base game. It was coincidental with the space DLC. It is here to stay and should be.

Sinking ships using hatches, sinks every time. https://youtu.be/ge4-z4YQ5O0

Sinking ships with door frames. https://youtu.be/4YMpgYviqb8

So you say, but when I open up a lower hatch on a ship--even when it's ventilated--the water barely trickles in.

It does better with doors on the sides, but still doesn't do all that well. There must be some trick to it.

Ehh, Due to the devs ruining alot of creations, Big ships cant really sink anymore unless you blow them up due to a update.

And that's why I rolled my game back to pre space. Best decision I ever made.