For me both can easily set up your win condition, but why I prefer the fish one is because its not grave dependent and going into this next meta there is going to be tons of graveyard hate. But first lets look at the beast build
Ok the guys a 4 star water which works with Moray and your shark. Though he is a 1500 meaning he's open to BTH but the reason for beast is to utilize the effect that SS a 4 or lower Gishki from the grave on normal summon. So he can target another beast and also an Ariel that we will talk about in a sec. Often times you may set him so that he can do a turn stall and then be ready for the next beast to come out
Sadly though running 3 Beast is not enough gishki to justify the engine so if you were to look at the other 4 star (for shark) waters (shark and moray) gishiki's (beast targets) you will find Gishki Ariel
So heres a cool one its a flip effect monster that on flip will search a gishki. Meaning you will have a beast now in hand with a gishki in the grave to go right into shark. You really gotta be careful though because its easy to read a set Ariel. So It may be beneficial to adjust your main or side deck to take into account your meta, fissure/macro hurts as does the destruction abilities of inzektors and agents (In which case Giski Avance may work better).
SO heres the build to mess with some with the added Gishkis
Other fun cards if you dont like the Gishki's
Cloudians cant be destroyed by battle so are good ones to stall for a turn Dragon can also be SS by removing a cloudian so it opens up some plays
This card cannot be Normal Summoned or Set. This card can only beSpecial Summoned by removing 1 WATER monster in your Graveyardfrom play. During each of your opponent's Standby Phases, you can change the battle position of 1 of your opponent's face-up Monster Cards. Once changed, the monster must remain in this position for the rest of the turn.
Spirit is cool in that in this deck its easy enough to ss her out like Cloudian Storm dragon it sucks having Monsters that can not be normal or ss unless by special conditions. The blow of this is cushioned by 3 morays but still its not optimum
Those familiar with bubble beat can see where this is going, creature swap and mystic box are both normals so can be set and still used that turn. The monster count is a little high to really make this work but again moray can aid in getting it to work. being able to add a rota too is nice especiall since you can then think about using Skull mariner as a target for Rabbit to go into Blade Armor or Excaliber
Its a funny one if theres a field of monsters and this guy is destroyed by battle you can keep summoning another out for the cost of 800 LP. SO hes a good stall and if you time it right you can end thier attack phase with your last face up panda so you can normal into a level 4 water and go into shark