The situation:

Version 1.3.2

Turn 312 167 Ships 1284 Maint cost

Turn 314 257 Ships 4097 Maint cost

I go looking for the cause.  I find that if I go into the diplomacy area and trade with another civ that I now have 33 new ships each of three different types (99 total ships).  They don't exist on the map, but I am paying for them.  Using the designer to determine main cost I find that those 99 ships are costing me 2914 per turn.  This has happened before, but this is the first time I have tracked down the cause.

I traded the phantom ships to another civ.  I hope they don't actually get them.

Thanks!