Until they come out with a waterproof iPhone that I can use in the dinghy when it is raining the apps are only semi useful to me but everyone's needs are different.
I use Navionics. I started with Motion X and still have it installed but prefer Navionics.
I use the compass app to confirm the vessel mounted compass is in the ballpark. Some large disparities have been observed.
I use Wind Meter. Not everything I get on has instrumentation and sometimes it's fun to see how close I can guess the windspeed.
Not an app but a waterproof cover that lets me talk, take photos and use all the apps in rainy conditions is a product made by Aqua Box. I've no affiliation but love this device and find it much more useful than the standard small Pelican Box. Here is a link Aqua Box - Waterproof protection for your mobile smart device