From the name I was expecting an archetype of LIGHT/Reptiles xD
These look well done. Some effects are basic, like the extra Normal Summons, or the discard effects of Diver and Snatcher, but their "when Normal Summoned, gains X effect" effects add a flavorful, archetype-feeling touch.
Seems you want to keep them in check with their "you can only Special Summon Sea Serpents" restrictions, but I wonder if that's too much and if you may loosen it up a bit without breaking anything. IDK really, since they do appear to be decent at swarming, especially with the Level 2 and 3 that can function as an engine of sorts due to their floating effects. For instance, using both for a Synchro or Link Summon nets you 2 searchers, almost like Goukis xD
I appreciate that these are somewhat compatible with Gishilnodon since it's a Level 5 Sea Serpent Synchro. Brionac is an option too, although they can't go for Synchro 6s without techs.
With some Level 7 members, maindeck, Synchro and Xyz monsters, they could do something with Danger! Nessie!. Just a thought.
Pretty sure Boglands needs some text fixes. I would write it like this:
During the turn player's Main Phase, they can Normal Summon 1 Sea Serpent monster in addition to your Normal Summon/Set. (This effect can only be gained once per turn.) [since it applies to both players] Once per turn: The turn player can return 1 Tribute Summoned Sea Serpent monster they control to the hand; that player Special Summons 1 Sea Serpent monster from their Deck. If a "Serplight" monster(s) you control would be destroyed by battle or card effect, you can return this card to your hand instead.
By the way, Swampland is a Field Spell. Not sure if that's intended, of it it's meant to be a Continuous Spell, or else it clashes with Boglands.