I definitely wouldn't disassemble the Apple Watch unless you have complete confidence it would not corrupt or ruin it. Otherwise it would likely void the warranty or worse, it wouldn't function normal if you couldn't repair it properly.
Also, I wouldn't subject the Watch to water or test to see if it is a gasket, I would rather have Apple inspect it and allow them to address the issue, if there is one. If there was an issue with the seal, then it would be a warranty concern.