Nice video. One can often scrounge a Fresnel lens off the front of a rear projection TV that is being discarded, I was toying with such a system to make a solar powered forge some years ago. As one can see from the setup, this is not likely something that is easily used on a boat - even a relatively stable catamaran
with lots of top deck
space (and, as demonstrated, the fire hazard is not to be underestimated).
Note that there are some nice designs for relatively low temperature flat plate solar stills more easily used on a boat - "Sailing the Farm" might have directions for making one if I remember correctly. A low technology and inexpensive watermaker
from the past, largely superseded by high pressure reverse osmosis
technology. Won't make a cup of hot tea though.
To get tea from the sun, one can just make 'sun tea' - a clear closed container containing potable water and some tea bags left in the sun to steep and left out after dark to cool (or perhaps immersed a couple of meters below the surface where the sea temperature is cooler). The slow steeping process leaves the tea without the often bitter tang obtained with boiling water when the bags are left in too long.