You probabably don't want to consider building your own sensor, unless you want to measure something rather exotic and building the sensor is a significant part of the project.
You don't say anything about the magnitude of force you want to measure, or if it is time dependent (and if so, what is the frequency range you are interested in).
Two standard methods are to use a load cell, or use strain gauges to measure the strain in a part of the structure that transmits the force (e.g. in a rod pushing on something).
The project is time dependent and it involves the force of water during swimming. So my idea is that I want to find a way to build my own sensor so if I would have to rebuild the multiple sensor I can.
For the magnitude of the force is what I want to test. I have no idea how much force is being used.