FreeGameDev

Judging by videos such as https://www.youtube.com/watch?app=desktop&v=W0dLll3IECA , Tux can jump to some height after swimming in water.

How do I do this? I tried spamming the jump button (Space) but nothing happens, the jump is still very low.

depends

to gain a decent ~6 tile amount of height from water you just need to build up the speed in the water by holding space

to gain a lot of height or exceed the swimming speedcap you need to fall from a high place, gain tonnes of momentum, and hold jump when you hit the water and hold a direction to gain speed. the downwards velocity is converted into horizontal velocity