Double skill weapons, +99s, accessories, some stronger eidos etc. Just a combination of a lot of things.
This is personal opinion, but I always level double skill LB0 gacha weapons and usually don't level pure assault ones, since the benefits between those and SR assaults are somewhat neglectable and I'd rather use the additional Rs and SRs it would take to max them on a different grid. Only exception I made was Ares axe, due to my lack of fire weapons at the time.
Your water team looks plenty strong already, I don't really see much reason to invest further into that team for now. Just max your grid, +99 your eidos / some weapons and level any good new water SSR weapons you get. Might be better off spending MT on other elements. Soul weapons are rather preference, depending on the playstyle and soul you want to use, so just pick whichever seems best to you. Attack weapons are usually prefered (cause they are ele atk + you just lose out on a lot of damage without the 30%) and popular ones seem to be Herc, Shingen and Morgan. For thunder often D'Art due to synergy with Mammon.
Progress wise, your weapon grids really could be better for that amount of time, slacker. :P