(No crash)
I tested with minimum mods to make sure. Whenever I put Simple Hats, I'm unable to swim in water.

Neoforge 1.21.73
accessories_cclayer-9.0.12-beta.19+1.21.1
accessories_tclayer-3.10.0-beta.18+1.21.1
accessories-neoforge-1.1.0-beta.16+1.21.1
architectury-13.0.8-neoforge
cloth-config-15.0.140-neoforge
connector-2.0.0-beta.3+1.21.1-full
ConnectorExtras-1.12.1+1.21.1
forgified-fabric-api-0.104.0+2.0.18+1.21.1
OctoLib-NEOFORGE-0.4.2
owo-lib-neoforge-0.12.15-beta.9+1.21
simplehats-fabric-1.21.1-0.3.2

Also tried with:

cloth-config-15.0.140-neoforge
connector-2.0.0-beta.3+1.21.1-full
ConnectorExtras-1.12.1+1.21.1
forgified-fabric-api-0.104.0+2.0.18+1.21.1
simplehats-fabric-1.21.1-0.3.2
trinkets-3.10.0

Unfortunately, it still doesn't work

I have the same problem. Same version and mods.

This appears to be a known issue with Sinytra Connector that seems to be triggered by multiple random mods.
(SimpleHats does nothing related to swimming or even movement/attributes so its not something for me to fix on my side)
Sinytra/Connector#1343
There is a workaround in that issue thread in the meantime while waiting for them to fix the issue.