Minecraft – How to /testfor if a player is sitting in a boat

You should run the following command before the /effect command, to make all entities with the tag "Detector" give all players within a radius of 10 a tooClose score of 1:

/execute @e[tag=Detector] ~ ~ ~ /scoreboard players set @a[r=10,rm=1] tooClose 1

And then reset the score after the /effect command has been run:

/scoreboard players set @a[score_tooClose_min=1] tooClose 0

You can replace r=10 with your detection radius, and tag=Detector with however you're identifying the mobs you are wanting to detect the player, or keep the tag and do something like to add the tag to mobs:

/scoreboard players tag @e[type=Zombie] add Detector

@a can detect dead players, so you use it instead of @e if all you want to detect is players:

/testfor @a[team=Yellow]