When you are in a boat, you mine at the same speed you would when you are underwater. The boat also appears partially submerged and filled with water, as in MCPE-91079. Also, when you are standing on a boat, you bob up and down more than you should, and air bubbles appear above the hotbar momentarily, in sync with the bobbing. This is shown in the videos below (they are two halves, the original was too large).