Most pools (all the ones round here have specific notices) will not let you use a normal disposable nappy (disposable swim nappies ok) because the gel that is in them comes out and clogs up all the filters - people might do it and not get caught but I know they do cause difficulties for pools - which I can completely understand after putting a disposable nappy through the washing machine - it makes a completely horrible sticky mess. We had reusable swim pants (had some 0-6 months which fitted from around 12 weeks) but did also use Huggies swim pants at 12 weeks without any problems.