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).
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We're tracking this issue as MCPE-83333, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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The mining speed penalty when you are sitting in a boat is due to your feet not being on the ground. Your mining speed would be the same while riding a horse or floating in the air. This is working as intended per MCPE-83333.
The underwater penalty is only triggered when your head is underwater. It stacks with the feet-off-the-ground penalty.
The water appear in the boat is due to a 3rd-party resource pack, according to MCPE-91079. Issues due to resource packs cannot be confirmed as bugs here on the tracker. You should follow up with the pack's creator.