I haven't tried it with a mob other than a zombie and a drowned but it seems like they can still hit a player when they're 2.5-3 blocks away when they're in a boat.

How to reproduce:

• Start a creative world.
• Spawn a zombie using a spawn egg.
• Place a boat and push the zombie into the boat on surface.
• Switch to survival mode.
• Try to get close to the zombie.

Observation:

The zombie will be able to hit you despite being about 3 blocks away from you.