It is really down to luck if you get decent weather or not, pretty much anytime from Mid Nov to Early April you can get hit with a cold snap. There are a few things to take into consideration as you weigh up the gamble. I would say you have an 80 per cent chance of good enough weather at that time of year, but I am designed for cold weather so others ( skinnier chappies) may disagree with that figure
. Check out the "average temps" for that time of year but I would think the highs are in the 75-80 range.
1) What feels FREEZING in summer when the air temp is in the 90-100 range doesn't feel quite such a nasty shock when the air and water temps are closer together. It's unlikely that you'll get weather that means the day is a complete blow out, it's just a question of how much you can make of the other attractions. At the very least there is the dolphin swim, some time in the aviary, lunch and hopefully a couple of visits to each of the coral reef and stingray lagoons.
2) DC is A LOT OF MONEY, but it does include a weeks worth of travel to Sea World. by making the most of those tickets, ie using 3 or 4 half days at SW coupled with other excursions everyone feels like they have had a "park day" but you're not left exhausted from trolling round any number of queues and rides all day. If it's a major issue, then I'd be tempted to leave it until I revisited at a warmer time.
3) You're coming from the cold UK, if you book your visit to DC within the first couple of days in Florida it is still likely to feel quite warm to you, even if it's only in the low 70s.
4) The smaller and younger any kids are, the more they will feel the cold and the less time they will be able to stay in relatively cool water. If they are under age 6 or quite skinny it is likely they will REALLY feel the cold. Their ability to swim is another factor. If they swim quite well, then it's likely they will keep warmer than if they just "bob about" in the pool.
I do think it's a shame that DC doesn't have some sort of contingency "shows" or "edutainment" they could put on in this situation (or summer thunderstorms for that matter) It would be really easy, IMHO, to have a small theatre/covered area where they could put on a bird/reptile display or "meet and greet". It would be a relatively low cost item, but would add value to the park on the occassions the weather puts a bit of a downer on the "regular" offerings. Two slots of half an hour each would really give people the feeling they had recieved their monies worth as it would give them something to do while getting warm enough to chance another dip in one of the pools.
Anyway I digress, in answer to your question " is it as good as they say it is? "
IMHO yes it is, but do keep a healthy dose of realism in your expectations as it can sometimes be difficult to live up to the highest hopes.