I would suggest slapping on a specially defensive Rain Dish Ludicolo, seeing as you have enough offensive pressure. Rain Dish Ludicolo is the bane of opposing water monotypes as it is crazily bulky and it basically can't even be scratched by them. Also granting you a grass neutrality and a x4 water resistence. However if you want a balance between specially defensive Ludicolo and an offensive one you could always try the last gen RU Offensive Tank, which tbh would fit pretty well on your team.
It is also a pretty much your best answer to Rotom-W which can be very threatening on your team at this point.
Offensive Tank
Ludicolo @ Life Orb/ Leftovers
Trait: Rain Dish
EVs: 160 HP / 252 SAtk / 96 Spe
Modest Nature
- Scald / Hydro Pump
- Giga Drain
- Ice Beam
- Synthesis / Substitute / Rain Dance
This set gives you a bulky all-purpose Pokemon for a monowater team, dealing with other monos with Giga Drain while also denting possible opposing grass monos ( however you stated you will only be facing other water monos) with Ice Beam. Scald / Hydro Pump, reliable move+burn chance or nuking STAB. Lastly ynthesis for recovery, however lefties+Rain Dish under rain is already pretty much enough. Substitute, for if you want to shield yourself from any possible damage and lastly Rain Dance, for just incase you would want to provide some rain for your team. However I doubt that is really necessary seeing as most water monos automatically provide rain for you.
Specially Defensive
Ludicolo @ Leftovers
Trait: Rain Dish
EVs: 248 HP / 204 SDef / 52 Spe
Calm Nature
- Scald
- Giga Drain
- Synthesis / Substitute
- Leech Seed
Pretty much like the one posted above, just specialized in being bulky and annoying. However I must say, this set does appreciate the rain a little more. This Ludicolo is pretty much the ToxicStall Gliscor under rain....just without toxic and more offensive pressure.
However, if you do wish to choose between a larger scale of Pokemon and have a better "wildcard", I would equally suggest a Pokemon. The first one being a based Toxicroak. It is by all norm that Toxicroak is by far not as effective as it once was in rain, however it is still a very potent threat under it, being capable if denting, to even sweeping teams under rain. It also helps against your grass weakness and is immune to water type attacks due to Dry Skin. Also a very decent fairy slayer with a rather interesting typing being perfect for a water mono.
Seeing as all of its gen 5 sets are outdated, I would highly recommend sets that must be crucially followed.
SD Croak
Toxicroak @ Black Sludge
Trait: Dry Skin
EVs: 28 HP / 248 Atk / 232 Spe
Adamant / Jolly Nature
- Swords Dance
- Drain Punch
- Poison Jab / Ice Punch
- Sucker Punch
After one Swords Dance, this set enables you to break through teams like a mad man. With the addition of fairy types, I would personally recommend Poison Jab over Ice Punch this gen as it is STAB and is generally of more utility in this new era. With Poison Jab, you will also have a very decent answer against grass type Pokemon that frequently appear on water monos, such as opposing Ludicolo or Azumarill, both being KOd after one Swords Dance
NastyWave
Toxicroak @ Black Sludge / Focus Sash / Life Orb
Trait: Dry Skin
EVs: 28 HP / 248 SAtk / 232 Spe
Modest / Timid Nature
- Nasty Plot
- Dark Pulse
- Sludge Wave
- Vacuum Wave
Yeah, it's deadly. Basically this set doesn't really care about a burn to sweep, which I would say is rather interesting. Toxicroak has a very decent special attack stat and a very interesting special movepool. After one Nasty Plot it will have a 594 special attack stat ( at level 100 with 31 SAtk IVs and Modest) and a 540 SAtk stat ( at level 100 with 31 SAtk Ivs and Timid) respectively. Vacuum Wave is a pretty decent special priority move, capable of getting rid of some Pokemon before they KO you. This set basically gives you general coverage and could function pretty decently as a wildcard on a monowater team, such as yours.
Other options could be SubSD, however that would really need the rain support.
This is personally the Pokemon that seems to fit best on your team imo.
Good Luck