Marvin has an Apple Watch app for checking off tasks and adding new ones from your wrist.
Getting started
Install Marvin on your watch from the Watch app on your iPhone. Make sure you're logged in to Marvin on your iPhone. It can take about a minute for your tasks to show up the first time.
Using it
Your chosen list shows on the watch.
Tap a task to mark it done. Tap again within 2 seconds to undo.
Tap + to add a new task by dictation or keyboard.
Tap the list icon to switch lists. Tap Choose... to add more favorite lists, swipe one to remove it.
Screenshot to add: Marvin on Apple Watch
Complications
Add Marvin to your watch face to open a list (like Today or Inbox) or jump straight to adding a task to a list.
If something isn't syncing
On your iPhone, open sidebar > Account & Settings > Apple Watch Troubleshooting and tap Sync now. It also shows when the watch last synced. Some settings, like category colors, sync every now and then rather than right away.