Minecraft – How to make the house boat move with command blocks

The full command you want should be this:

/execute @p ~ ~ ~ /setblock ~ ~ ~ minecraft:dispenser 2 keep {Items:[{id:35, Slot:0, Count:-1,Data:0}]}

The "execute" command takes a selector, a set of coordinates, and another command. It runs that other command as if the selected entity was running it. For example:

/execute @p ~ ~ ~ /say hello

Would make the closest player say "hello", even if a command block or another player is actually running the command above.

When they run over the "start" pressure plates have a scoreboard objective set

/scoreboard players set @p <objective name> 1

And have a clock running all the time doing this command

/execute @p[score_<objective name>_min=1] ~ ~-1 ~ setblock ~ ~ ~ air

The execute @p[score__min=1] means "Do this command relative to @p with a score of one in that objective" And the setblock sets the block under the player to air.

Hopefully this answers your question.