Q:

Dipole Sleeve Antenna design in fresh water - Transmission frequency and emitted power?

Hello dear antenna enthusiast,
I would like to receive and emit EM signals with a dipole antenna in the range of **433.9 to 434.5 MHz in fresh water.
So I chose a half-wavelength dipole sleeve-antenna design that I made by myself out of coax cable (part of the coax is folded back and acts as the counter pole.
So one pole is the inner conductor of the coax cable and the counter pole is the outer layer of the coax stripped back and soldered onto the coax cable in the middle of the antenna. Similar to the one in the video...
https://www.youtube.com/watch?v=hK0KX0YcvH0
However I calculated the 433 MHz half-wavelength in fresh water as round about 19mm, but when I compare several antennas of the same type with different pole length, the one that has a peak in the reflected power plot (SWR in dB) at 433 MHz has a pole length of 80mm and with shorter poles the peak moves to the HF-area (as expected). You can see one of the reflected power plots in dB below, the antenna was in the middle of a 4mx8mx2m fresh-water tank connected to a VA by a 4m thin coax cable.

So I have now following questions...

Why is one antenna pole 80mm instead of 19mm, what would you expect?
Do I have to look at another physical value with my Network Analyzer or other gadget to be able to judge the performance of my antenna (max. emitted power), how does normal values look like? 
Is it typical for a Dipole to have only one SWR peak at one frequency?

A:

Unfortunately, there are some problems with your ask.
First, that video doesn't detail the construction of a half-wave dipole, but a monopole with a 1/4 wave sleeve around the coax feed to decouple the line. Mounting them can be problematic since none of the conducting surfaces are at ground potential. In other words, you can't just bolt the bottom section to a mast with metal fasteners. 
Just because you solder the other half to a piece of metal doesn't make it an effective second arm of the antenna (e.g. dipole). It's (1) aligned with the feed line and (2) completely surrounds it at equal potential. This won't radiate in the far-field. What this structure actually does is load the feed point of the antenna allowing it to look electrically longer or shorter than it physically is (see "antenna tuning" or "antenna matching") and presents a virtual ground impedance very close to the feed point making the precise location/orientation of a real solid ground plane less critical.
Submerged Antennas...
Second, transmission in fresh water doesn't work the way you've described because the water polarizes and therefore the attenuation factors are extremely high and your dielectric coefficient goes way off. Most transmissions of "underwater" antennas don't actually propagate through the water, but above the water (or at the air-water interface) by radiating off of the return path (outer conductor) of the cable... and this is a low-efficiency transmission obviously. 
Or... because the air pathway is so much less attenuating, any energy is more likely to go "up-over-and-down" (or "down-over-and-up") rather than directly, even at short distances.
Here's a figure from my book that illustrates this with the relative attenuation factors:

