Watthydra is one of the more interesting Synchro monsters out there.  We have a Light attributed, Level 7, Thunder type Synchro monster, who only has 1500 attack (and defense.)  Watthydra is Synchro Summoned specifically with a Watt Tuner and one or more non-Tuner, Thunder-type monsters.
 
Watthydra can attack your opponent directly, which is good, but there certainly are better and easier monsters and ways to directly attack, which also can be done without a Synchro Summon.  Now, to save on a bunch of text here in short, the second effect says: 
 
"When I successfully attack your oponent directly, I give you the exact effect of Different Dimension Capsule."  Just like said Magic card, you remove a card from your Deck from play, and during the second Standby Phase after activating, the card comes to your Hand.  Not really all that worth it I'm afraid...
 
Ratings:
 
Traditional:  1/5 
Advanced:     2/5 
Art:  3.5/5

Hello there COTD readers. Today we have one of my personal favorites.

Watthyrdra is a level 7 LIGHT Thunder type monster that requires a Watt tuner and 1 or more thunder type non tuner monsters. So this thing is specific, but it is a watt, so obviously your only going to use him in a watt deck. So what does he do? Well he has 1500 attack and can attack directly, like most watts can. When he inflicts battle damage through a direct attack, you select one card from your deck and remove it from play, its returned to your hand during the 2nd standby phase after his effects activation.

So, hes gold sarc on legs. Thats actually pretty good for watts since they dont have much draw or search power. Quite a good card.

Traditional-1/5
Advanced- 4/5 (Watt Decks only)
Art- An electric hydra. BLAH.

Watthydra is a pretty cool card for the Watt theme, but unfortunately suffers from some glaring flaws.
 
The good news is, it's the largest Watt monster yet, with 1500 ATK. It can attack directly, and when it hits your opponent, you remove any card in your deck from play, adding it to your hand during your second Standby Phase afterwards. This adds an excellent source of damage and consistency to the Watt theme, and it's very welcome.
 
Now the bad news. Firstly, it has 1500 ATK. Now, normally one would want all the ATK one could get on a direct attacker. In certain Watt variants, this is awesome, but in the versions using Messenger of Peace, you won't be able to use Watthydra to its fullest extent.
Secondly, it's a level 7 Synchro Monster. Because of the way Watts are leveled, you'll be forced to put three monsters into making the Watthydra, or you have to use subpar monsters like Wattkiwi.
 
Overall, Watthydra is still a welcome addition to the theme, and when it comes up, he'll be a great benefit if you summon it. I wouldn't want to build a deck around him though, as you're stuck using more of the bad Watts. Leave some in the Extra Deck for a "just in case"
scenario.
 
Traditional: 1/5 Watts don't FTK. Advanced: 3/5 Watts aren't too shabby really, but Watthydra doesn't do enough to help them out, since they rarely can even summon him.