The best mermaid paintings show the beauty, mystery and sensuality of the mermaid. It is also good if the painter has done his homework and the picture has a mythic element to it as well. This may come through better in a more abstract painting. Mermaids after all are fantasy creatures and a realistic rendering may just get in the way.
Mermaid paintings have been done by pin-up artists and the great masters. The mermaid means different things to different people but the symbol of the mermaid remains. She is a classic subject reflecting the mind and times of the artist.