Some softwares have options for speed calculation. If you use a simple camera at the side of swimming pool you could insert some points with know distance and then estimate the average speed, using two tor more judges. 

You need a camera capable shooting with 60 or 120fps. Go Pro, IPhone 6, Casio Exilim, Samsung Note 3, 4 etc. suitable for you. You can measure the speed with Kinovea free movement analyser software.

There is one more option to measure speed. You can buy the following laser distance and speed measuring tool:

http://www.ebay.com/itm/6x24-Waterproof-Laser-Range-finder-Hunting-scope-Distance-Meter-and-Speed-Finder-/191666319290?hash=item2ca033efba:g:GOkAAOSwPgxVMLZy

@Béres Sándor. 

Thanks for your reply, Are you sure, can use this device for ball speed?

are there any validity and reliability for this device in sport 

I think scientific point of view the slow-mo cam and motion analyser softwrare is suiatble, but during the training the laser speed meter can be better. But it should the case occur you have to ask the seller that the device work with balls too. I have never tried it in ballgame.

Have you considered using a radar?

Sport Radar gun

@Dimitrios Skoufas

Thanks for your answer  but I want to capture video and then realize the speed 

I do not have radar gun

For calculation of speed from video taken by standard video camera will not be helpful. The throw of ball will take less than 3 sec. Standard video camera record at 30 fps and you get a low resolution. It is best to get a high speed video camera that can capture at a speed more than 200 fps. You have to calibrate the camera lens and the field of view. Try reading Theoretical foundation at http://www.kwon3d.com/theories.html

Good luck. 

Getting velocity off of any kind of video or film requires specific set up and filming capabilities. The most simplistic method would be to attempt to capture the ball from the instant of release until about .1 - .3 sec after release. The problem is the requirement of a distance reference within the plane of movement and since it appears that you are trying to get it off of what would amount to "game film" the results would be subject to more error than would be acceptable in a scientific paper. If it is just to get "ball park estimates" of velocity for coaching that would probably be acceptable. But throwing, especially water polo, under game conditions, is highly questionable in terms of valid and reliable estimates of velocity. There are rules for cinematographic analysis of movements and the requirements of time and distance reference standards within the plane of movement are not negotiable for scientific measurements.

If you know the size of the ball, the framerate of the film and the angle of the shot the speed can precisely calculate. The weakest chain to be known the angle of the shot to the camera.