Hi
If water remains in the speaker on your Apple Watch Series 3 after engaging and then turning off Water Lock, then it may help to repeat the process by manually enabling and turning off Water Lock at least once more:
About Apple Watch water resistance - Apple Support
If the speaker remains muffled after multiple uses of Water Lock, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for advice and assistance:
More information: