Nintendo switch has a bug when going under water the screen goes black texture pack is called chroma hills and also doesn't work on the higher resolution settings so you have to turn it down and also the resolution has to be turned down for purebdcraft not enough memory but the switch has 4 gigs that's plenty enough to run them both
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-32799.
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This bug can also cause the game to crash when joining a realm that has the chroma hills and purebdcraft texture packs enabled at max resolution.