Personally I would say "My house is in Kamakura, at the seaside" or something - it's not "the sea of Kamakura" like you say "the Sea of Japan" - it's not a proper name.
"Peaceful" and "beautiful" are fine.
I'll just do a version "par acquit de conscience" though I've had 6 hours sleep which isn't much for me. I'm not saying your version is wrong - I haven't really read it carefully to tell ya the truth - so pick what you like.
Watashi no uchi wa kama kura no umi no soba ni arimasu.
My home is near the sea at Kamakura.
[I don't think it is "next to" or your house would get washed away]
Totemo shizuka na tokoro desu.
It is a very peaceful place.
Kyoo wa amari samuku arimasen deshita. Sore de, ane to issho ni kaigan e ikimashita.
It wasn't too cold today, so I went to the sea shore with my [big] sister.
[I don't like the use of "sore" de there very much but that may just be a style thing... Also you'd probably use a different verb to "iku" normally]
Sora to umi ga totemo kirei deshita.
The sea and sky were very beautiful.
[Normally in English you use that order]
Shyashin o takusan torimashita.
I [or "we", it doesn't really matter] took a lot of photos.
Kyoo wa doyoobi deshita kedo, hito wa amari imasen deshita.
There were not many people about, even though today was Saturday.
Ookii kino shita de obentoo o tabemashita.
We ate our packed lunch underneath a big tree.
I've been to Kamakura, it's really nice. I think it may have been the capital under the
Minamoto family? There is a big buddha which looks cool, and lots of trees.