So far, I've tried:
Marine Chart, from EarthNC - not bad.
Transas - a bit too simple, not a real chartplotter.
INavX - my favourite, from the same people as the MacENC I have for N American waters.
Plan2Nav - a big disappointment, I'd expected better from C-Map. It crashes frequently.
However, I see that more are coming out. There is now a Navionics app, which just might be worth looking at.