There are a million apps out there but here is my list of the best ones that are free. Some come as more advanced paid options. By Category the best are
The weather for where you are and where you want to be. Classic wonderful.
Marine Weather by Bluefin
Excellent on the water weather! Gives marine forcasts and warnings
Wind speed, forecasts and history at thousands of spots around the globe. Real time snapshots of local wind conditions. A must have!
Simple but useful: Tide times at lots of locations and sunrise sunset.
On the water
Speed, log , GPS position, red backlight option. Great for when down below off watch and want to check how the boat is doing.
Oh no is that container ship heading your way? All the big container ships and many smaller cool boats use AIS ( automatic identification system) this tracks all of the boats that use AIS and allow you to see where they are going and identify the boats.