You can lift and divide 2 groups of pond plants in late Spring. Those that have finished flowering for the year already - Caltha species and those not due to flower until the end of the year - August/ September time. This would include Pontederia, Typha and Mentha cervina species.
It has been a late Spring this year and dependent on where in the country you are you may need to wait for a bit longer until your Caltha have finished flowering.
Remember that pond plants need to be repotted into mesh baskets using aquatic compost (which you will need to get locally as it is too heavy for us to send through the parcel system) so have a supply of the necessary equipment ready before you take the plants out of the water. They cannot remain out of the water long or their wet stems and young leaf growth will shrivel up in the air.
Pond plants like Iris should not be divided now as this will stop them flowering this year. They should be divided, if necessary, immediately after they have finished flowering.