I created command block system that makes boat move on its own (by teleporting it), at the same time teleporting player to the position of boat (I wanted it to look as if player was in a boat).
However, the result was different than expected - boat seemed to stay quite far behind player, despite player being teleported to the exact position of boat, as if it was delayed.

All this can be seen on video attached
(sorry for low quality, 10 Mb just isn't enough for good quality videos, and version of this one that looked only a little better was already around 12 Mb as far as I remember)

Command used to teleport boat:

execute as a72159cf-7f9c-41a8-a555-15365eb5ca52 at @s if entity @a[distance=..0.5,tag=RideActivated] positioned 2416 62.5 391 unless entity @s[distance=..0.2] at @s run tp @s ~0.2 ~ ~
yellow part - marks my boat as target entity
red part - ensures that boat doesn't start unless there's adequate player nearby
green part - stops the boat when it reaches its destination
blue part - makes boat move forward, that's part of command most important for this report

Command used to teleport player:

execute as @a[tag=RideActivated] positioned 2415.7 62.5 391 unless entity @s[distance=..0.3] at a72159cf-7f9c-41a8-a555-15365eb5ca52 run tp @s ~ ~ ~
yellow part - marks player with tag as target entity
green part - stops player from following boat after they reached destination (because then boat returns to start location, without player)
blue part - teleports player to position of boat, that's part of command most important for this report

a72159cf-7f9c-41a8-a555-15365eb5ca52 is my boat, sorry if commands don't look transparent, I copied them the way I used them.