Some pond plants outgrowing their baskets can be lifted and divided now but not those due to flower before June like Caltha or Iris species. Do those immediately after flowering.
See our Tips and Advice page on propagating pond plants. Use surplus plant material to make more baskets so that in years to come you can divide one basket in alternate years to another. Use aquatic compost and never a peat based multipurpose compost and also a mesh basket so that the plant roots can leave the basket and surf the water for nutrients. Pond plants do not like their roots controlled within a tight space and this will cause the roots to rot over time.
Try and remember if your waterlilies were sending leaves vertically up and out of the water last year – a sure sign they needed dividing. (Photo the pond in the summer to help you remember which to divide later).
Lift the baskets if you can or cut off the rhizomes with a knife and leave the mature basket where it is. The effect will be the same – you will remove a number of shoots and that will thin out the congestion of growth coming from the basket centre. See or Tips and Advice page: Repotting a waterlily.