A badkruka is someone who dislikes or refuses to swim, usually in an open body of water and often due to the low temperature of said water.
The word comes from bada ('to swim' – it shares roots with the English verb 'to bathe') and kruka, which literally means 'pot' or 'jug' but is also a slang term for 'coward'. So you could translate it as 'bathing coward'.
This brave pup is anything but a badkruka. via GIPHY
As you might have noticed if you've ever spent the summer in Sweden, you're usually expected to spend a large portion of the season outdoors and engaged in active pursuits: hiking, climbing, and swimming.
These activities are typically done based on the time of year rather than the weather, so you'll find many Swedes leaping into extremely cold water just because 'it's summer', and using badkruka as a good-natured insult aimed at those who refuse to join them.
Note than in Swedish, the verb simma (which also means 'to swim') is usually used when the focus is on the movement, for example when describing athletes who are racing or someone swimming from one side of a lake to the other. Bada usually refers to more recreational swimming, for example someone who jumps into a lake but swims around without any clear destination.
Bada can also be used in the same way as English 'bathe', describing someone in a bath or body of water, with the aim of cleaning themselves or recreation.
As for why kruka means 'coward', it actually comes from a much older Swedish word, kruker, which also meant coward and came from a dialectal verb kruka, meaning 'to bend over/to crouch'.
Hoppa i, badkruka!
Jump in, scaredy-cat!
Jag är ingen badkruka, jag älskar att bada
I'm not scared of the water, I love to swim