Best undergloves/liners IMHO are the Fourth Element GL1 neoprene glove liners. Plenty of dexterity and when the gloves leak your hands are kept warm.Liners can probably be had pretty cheap. Thin fleece or wool gloves should work for 50F water.
For colder water in the winter, add on some wrist warmers too. For really cold, use thicker drygloves such as the textured ones from Kubi or the blue 'Smurf' gloves. These are dependent upon the actual glove rings and how they work.