It is such a beautiful day that I’d like to go fishing in the river.
素晴らしい（すばらしい）日 sounds very unnatural in this context. We hardly say 素晴らしい日 except when you mean “it’s a special/memorable/honorable day” such as a wedding or award ceremony day. Therefore, “とてもすばらしい日なので川に魚釣りに行きたい” suggests that the speaker accomplished something great today, and he wants to go fishing to celebrate his achievement (i.e. he indulges himself and is on a well-earned holiday).
I believe “It is such a beautiful day” in English means “the weather is perfect today”. My alternative translations are:
今日はお天気だし、川に釣りに行きたいな。（きょうはおてんきだし、かわにつりにいきたいな）= (lit.) The weather is sunny today, so I want to go fishing in the river.
今日は釣り日和だから、川に行こうかな。（きょうはつりびよりだから、かわにいこうかな）= (lit.) Today is a perfect day for fishing, so I’m thinking of going to the river.
お天気だ means “the weather is great”. お is often conjugated with a noun to make it polite. But this is not the case, but an idiom.
XX日和 means “the day is perfect for doing XX”. 釣り日和 means “perfect day for fishing”, and 洗濯日和 (せんたくびより) is “perfect day for laundry” (i.e. sunny, not so humid, not so windy). 日和 consists of 日 (day) and 和 (peaceful; amiable) ==> the day perfectly matches your objective.
魚釣り and 釣り are completely interchangeable. You can use both in formal and casual situations.