I've been using a sextant
since the 1960s. With a full sized sextant I can get within a mile on a stable platform like a ship. I have two of the small ones I use for decoration. I tried one at a dock
with a clear horizon, I can only get about a 3 mile circle in good viewing conditions. I was told the small ones were made and sold to meet a personal equipment
requirement in some merchant services.
I suppose 3-5 miles would be ok in the middle of the ocean. Even with a good sextant, you need to practice to get a decent fix.