for prosperity? the extra deck isnt all the important for this deck but probably levia, no. 103, one of the kragen spawns, mistar boy, reprodocus and unicorn are the least useful. do note that there are better cards to be played like bahamut shark and toad but I haven't had the CP to craft them yet

you're trying to turbo out kairyu shin with an umi on the field to lock your opponent out of summon non-water monsters, while using cards like sea stealth attack and jellyfish to protect them, if you don't have full access to your engine you also have tcboo as a backup plan to sit on. mega fortress whale help you close up games and raiho is just a really good anti recover card in a grind game. there is a guide on the deck here on the website so you can take a look at that

its pretty good, very good anti-recovery. its not really that hard of a brick either since jellyfish can summon it from the hand, and some decks just cant play through it.

lul