If I deactivate a user will I lose their data?

No, you will not lose their data. Press Actions > Mark as inactive then you can choose to reassign their data to another user now or reassign it later. 

The deactivated user will appear in that section and have a button next to it to reassign the data. When you reassign data, all previous activities and task or events that have been completed will stay under that deactivated user's name. Outstanding tasks, events, deals, etc will be reassigned. 

If you do not reassign the data, you can filter the list views to see the information under the deactivated users. Everything else will just stay as is (activities, tasks, events, etc).