Water is not a problem (plenty of options along the way) and the ferries over Hardanger go regularly (both free). There are no tunnels until you reach Hardanger. Then you have quite a few heading to Bergen. The section from Geilo to Haugastøl on road 7 is the only main road you need to cycle. It’s 25km and can be skipped with the train. You will find more of my thoughts and suggestions on the video I mentioned in the link above.