What do you do in the summer? When do you usually go to the sea? 

Where do we/they usually go in summer?

When do we/they usually go to the seaside?

Who usually goes to the seaside in summer?

We usuakky go to the seaside in summer, don`t we?

Do we/they usually go to the seaside in summer? 

Я писала we (мы) /they (они), потому что можно использовать и тот и другой вариант