Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Try a prolonged drying-out. It got thoroughly wet, even if only on the outside, after all.
Dry it out, unused, for at least 2 or 3 days in a dry warm place.
Then put it on and try it again.
This approach may actually help, since the battery compartment (and thus presumably the interior of the device) was not gotten wet, you know - if it had, you wouldn't have any device function at all. Instead, you have device function for sure, but it's a little off. Since it monitors your heart rate through external contact, if it were even slightly damp in places on its surface, this might be impeded and a lower-than-expected heart rate be measured as a result.
(If this answers your question, please rate my answer. Thanks.)