There can be a huge shift after an Olympic Games with many leading swimmers retiring.
Michael Phelps will be particularly missed at the World Championships because he excelled in so many events.
In this country we will be sorry that Rebecca Adlington is no longer competing in the pool, but she will be alongside me in the BBC commentary booth. It will be very interesting to hear her take on events.
The United States will still dominate but it will be fascinating to see how Germany and especially Australia respond to very poor Olympics performances last summer.
The Germans used to be very strong but now struggle to get competitors on the podium while Australia will want to put their horrendous Games behind them as quickly as possible.
Brits in Barcelona
Jazz Carlin could be in for a big world championships after making an impact and being there or thereabouts last year.
She didn't quite deliver at the Olympic trials but has made good improvements this year. With Adlington having retired she is ready to step up a level in Becky's favourite events and Jazz has world rankings in the 400metres, 800m and 1,500m freestyle.
Michael Jamieson, who won 200m breaststroke silver in London, is a gold medal contender in a discipline in which we have enjoyed success with David Wilkie, Adrian Moorhouse, Duncan Goodhew, James Gibson and many others down the years.
Michael will definitely be on the podium - it just depends whether it is his day or not. Only a world record by Hungary's Daniel Gyurta denied him in London and Michael could easily reverse the positions in Barcelona.
Hannah Miley was agonisingly close to a 400m individual medley medal in London and knows that had she swum her best she would have been on the podium. She will seek to put that frustration behind her, as will Fran Halsall who found herself in a similar position.
Fran did some soul-searching and changed coaches so look out for her in the 50m freestyle and 50m butterfly.
Ben Proud, Chris Walker-Hebborn, Craig McNally and Lauren Quigley are other new names not to miss. Hopefully they will learn from the experience and enjoy the wow factor of competing in a major championship.
Can Lochte fill Phelps' flippers...
America's Ryan Lochte raised a few eyebrows by putting his Barcelona preparations to one side to star in his own reality show in the United States.
His coach was not best pleased but Ryan enjoyed a lot of success at the Olympics and is a real character.
As long as he turns up at Barcelona and produces his best, which I am sure he will, I can't see the harm in giving himself and the sport a wonderful boost by becoming known to a wider audience.
Ian Thorpe retired too young and perhaps should have taken a year out instead of leaving it too late to try to make a successful return to the pool.
If you are going to have some fun, as Ryan has, the year after an Olympics is the best time to do it. I am sure he will be fully focused when it is time to get his act together for Rio.
Others to look out for
Missy Franklin created waves by winning four golds for the United States in London and Ruta Meilutyte, the Lithuanian 15-year-old based in Plymouth - and therefore claimed as one of ours - is now a year older and should do well.
Don't forget to watch out for Chad Le Clos too. I remember interviewing his dad Bert with Claire Balding at the Olympics when his emotional response to his son beating Michael Phelps to gold in the 200m butterfly became an international phenomenon.