Why not? I doubt there are any legal restrictions other than don't be a hazard to navigation. But it would probably be too small to be a hazard anyway, and likely too small to be noticed.
I happen to drop equipment into the sea for a living on occasion, and we always paint it in garish yellow for visibility and label it clearly with contact info to allay suspicions. Typically "Scientific instrument - flux capacitor, Acme Co. - www.bar.org // foo@bar.org // 555-2368"
Also, we notify the coastal administration and the mapping agency so that an EfS (NOTMAR; Notices for Mariners) may be issued if they deem it necessary.
A cynical colleague suggested we had better label them 'Highly radioactive' to keep fishermen from picking them up and taking them apart out of curiosity...