Your deck is full of a lot of singles and sub-standard cards. I've shown what should come out, and given some suggestions as to what to add, but this isn't a final list - you'll have to work out which cards you want to use yourself. I've suggested cards based on a control deck using Codarus, but different decks will need slightly different cards. It should be clear which of yours may be useful elsewhere.
-1 mermaid knight (Mediocre)
-1 maiden of the aqua (Bad)
-1 star boy (You can try it, but there are better options)
-1 unshaven angler (You only need this when you don't have Legendary Ocean out, and then for one monster. You don't need it)
-1 gagagigo (Bad)
-1 Giga Gagagigo (If you're using Level Limit/Gravity Bind, no. If not, they you can try it)
-1 ocean dragon lord - neo-daedalus (Too hard to use)
-2 fenrir (You can try one instead of Aqua Spirit, but it can be dead early)
-1 neo aqua madoor (you don't need walls)
-1 amphibious bugroth mk-3 (You can use this over Codarus, but I'm suggesting a different build.)
-1 nightmare penguin (You don't need it)
-1 the legendaru fisherman (bad)
-2 magical mallet (There are options that leave you with more cards than this)
-1 riryoku (Bad)
-1 big wave small wave (Unneeded)
-1 call of the haunted (unneeded with Monster Reborn)
-1 gravity bind (Limited - you can only have one in the deck)
-1 penalty game (Doesn't help your deck)
-1 tornado wall (Not really much use with Codarus. You could keep with Bugroth)
-1 xing zhen hu (Not needed)
-1 spell shield type-8 (There are more general options)
+1-2 codarus (Great for getting rid of things, especially if they can't be destroyed, since it doesn't destroy)
3x mother grizzly
+1 mobius the frost monarch (Good)
+1 aqua spirit (Only if you add the second Mobius. It's free tribute material)
+ Mermaid Archer (Gets under Level Limit even without the field spell)
+ Spined Gillman (Like Star Boy, but has a higher attack at the cost of not affecting a few monsters)
+ Terrorking Salmon (Instead of Giga Gagagigo, if you're using Gillman)
+ Deep Sea Dive (gets instant Lv5 Synchro or 2000 attack Mermaid Archer
+ Armed Sea Hunter (Stops effects)
+ Snowman Eater (Monster Destruction, good defence, M. Archer fuel)
+ Penguin Soldier (Good effect, if you flip it yourself you can return it to re-use it's effect
+2 Moray of Greed (Leaves you with the same number of cards, unlike Magical Mallet
+1 fissure/Smashing Ground (Either, although Smashing is usually more useful, if you feel you need more monster removal
+1 salvage
+1 mystical space typhoon (Now at 2)
+1 Dark Hole (Staple)
+ Pseudo Space (If you run 3 Codarus, gets more use from it's effect)
+2 Bottomless Trap Hole
+1 Mirror Force
+1 Solemn Judgement
+2 Dimensional Prison (All staple traps)
+1-3 Forgotten Temple of the Deep (If you're using Codarus, for more use of it's effect. Also protects your monsters from your Dark Hole/Torrential Tribute