I do know you mustn't use peat based compost but heavy clay loam without fertilizers or weed killer. Yes use gravel on top. They sound as if they need dividing.

You should use aquatic soil to re pot, but in the past I have just used the gravel. Garden soil will spoil the pond water and poison the frogs & wildlife oh and if your pond is big you can plant in the blue plastic mushroom trays. Or best get the aquatic pots from maidenhead aquatics or your nearest garden centre they don't cost much.

I usually just put them in to gravel and the roots take out the nutrients they need. Aquatic soil is lower in nutrients as already said and if you have fish, the gravel just stops them pulling the plants out. most ponds don't have 'flowing water' so the soil isn't really needed as anchorage into mud isn't needed.

any plantlets can be removed and 'potted on'. What plants are they just out of curiosity?

From where do the plants take the nutrients? from the water ?
The one with masses of roots is purple lobelia
the one with fat red shoots and plantlets [which are under water] is Houttuynia I think it is sometimes called orange peel plant

yes foxhaven the nutrients are dissolved in the water and enter the plants this way. Houttuynia roots easily and can become a bit of a thug. the lobellia will have a central crown from where the roots come and as it crows you get more 'crowns'. next year there will be extra crowns too and you can split it again. I'd be tempted to keep some in pots in shallow water rather than fully submerged. I do that with lobellia cardinalis.