I tried to give a score to players with a specific status effect as well as testing for players with a specific status effect.

However, the commands I used don't ever find a player with the specified NBT Data. /testfor doesn't work as well.

Further testing showed it is the same with mobs.

The commands are:

/scoreboard players set @a canSwim {ActiveEffects:[{Id:17}]}
/testfor @a {ActiveEffects:[{Id:17}]}

Commands for testing purposes:

/scoreboard players set @a canSwim 1 {ActiveEffects:[{Id:17,Amplifier:0,Duration:595,Ambient:0,ShowParticles:1}]}
/testfor @a {ActiveEffects:[{Id:17,Amplifier:0,Duration:595,Ambient:0,ShowParticles:1}]}

EDIT
Since, as Aaron Rhodes said, it might be confusing to see a score name of canSwim and the hunger effect in one command:

I was trying to create a permanent world rule which only lets you swim or dive if you have the hunger effect on you. Otherwise it kills the player trying. It is a part of an ongoing series on YouTube.

EDIT2
Alright, I found a solution, I needed to use "17b" instead of "17". You need to specify the type of the tag. I am not sure if it is intended, however, it is not the bug I posted this issue about. Thank you!

Commands:

/scoreboard players set @a canSwim {ActiveEffects:[{Id:17b}]}
/testfor @a {ActiveEffects:[{Id:17b}]}