I go with Seafaring civs, so get the extra movement and lower odds for sinking, plus play on archipelago and continent maps, so I need to explore a lot. I assume that one turn in the sea/ocean is okay, and two turns is just under 50% survival, going past that is when I loose them, as at 4 turns, the survival rate is under 10%. Problem is, bringing them back from 2 turns out is only a little safer than keeping going. Exploration is risky, just ask Henry Hudson and Bartholomew Diaz, Henry disappeared following a mutiny in Hudson Bay, and Diaz vanished without a trace while heading for India as part of a Portuguese settlement convoy. Then of course, you have the Franklin Expedition.
I am going to have to give thoughts to naming units, as it is a good way to keep track of them. More likely to do it with ships than land units, although naming armies might be a good idea. Have to check on naming armies. Need to avoid Seventh Cavalry and Light Brigade for cavalry units.