Coastal area ? what coast are you speaking about you do understand that in winter the land scape changes completely with icecaps growing by hundred of miles and that the supposed cost area becomes hundreds if not thousands of miles away of the liquid water? so anything related to fishing or tyde energy is impossible not only you aren't answering the energy issue we are talking about here I have the feeling that you are really underestimating Antarctica (and why scientist are having a hard time there despite the millions of dollars budgets)
here you go :
Also when it comes to temperatures, McMurdo antarctic base which is coastal and located in one of the warmest if not the warmest area in Antarctica the temperature can reach -50°C -60°C, the winter doesn't hit in one go, hence the mean temperature is as high as -28°C but that doesn't mean that there aren't days where the temperatures as low -60°C, as for winds blizzards and such, please, .... Antarctica is very windy place, since there aren't much mountains to block the wind(the continent mountains are under ice)
As for biogas.....let's discuss some facts so you know what you are talking about.....You need a 35°C in the digester to decompose waste into biogas.
Biogas is very hard to store as it is very hard to liquify but more importantly 50Cows, would produce the equivalent of 50L of fuel everyday, I don't know if you can put it in perspective, the amount of space and energy needed to give 50 cows a living environnement in Antartica would use more than those 50L of fuel per day I'm not even talking about feeding them (you can also use human waste and organic food waste but it's no where near what cows produce) so no biogas is not an option here. maybe you have other solution you didn't present yet so feel free to share
While you do have a point that the ice expands during the winters, but then, that just means that our fishing fleet will have to move further away from land. Pretty simple as well.
Wave power might be a problem in the winters, but we'd just have to compensate that with more wind power and biogas.
I can neither see the point that on that "some days it can go as low as -50°C to -60°C". The average temprature is mid winter −26°C. That's what we have to start from.
Thanks to all that wind we can get energy btw. The turbines could become covered in snow and ice during these blizzards, but when that happens we'd just have to go on our backup system with stored energy. When the blizzard is over we'd just have to remove the ice. The population needs to do something anyway.
Cows don't need very high temperatures, if we can keep the place on about 18°C it will work. And keeping something about 18°C under ground, especially with cows that themselves emit heat, is not that hard.
And then we'd just need to build the digester deep under the ground, heat it up, and there, no problems.