The Southold Bay Constables patrol the waters off Fishers Island, but I believe the Constables on the island are Fishers Island constables. As far as I know Southold does not have any constables other than the bay constables.
They may also have a dynamic talkgroup on the east Hampton P25 system, I have been hearing a TG (65420) which I am still working on verifying but may in-fact be southold. If anyone who can monitor east Hamptons system help to verify I would appreciate it. I understand these talkgoups arn’t permanent and come and go as needed but have found some of these talkgroups are in fact semi permanent such as the Suffolk co PD command patch and the sheriff 1 patch. As far as the Suffolk system the southold TG I have mostly heard on the montauk site.
Sheriff-1 and Command aren't patched with EH800. They have a shared ISSI "roaming" agreement. Certain talk groups like Command, Sheriff-1 work on EH800, and certain EHPD talk groups such as EHTPD-DISP can roam onto the County trunked system. On a side note, there are separate talk groups on the EH800 system such as SHERIFF-E and PD-E available to use if needed.