这是一个美丽的一天  （在这里，一天 不能用量词“一个”修饰，而且在中文里，一般不用完整的句子做标题。we cannot put the  measure word “一个” in front of“一天”。美丽的一天 means "a beautiful day" , and that in chinese, a  complete sentence isn't usually used as a title ）

The sun is shining brightly
艳阳高照 明亮地 太阳明亮地照耀着。

I decided to go for a swim in the lake.
我决定 去湖里游泳 在湖

The water was cold, but refreshing.
水是冷 or 水是冷的 却 or 水凉爽 水有点冷，但却令人舒爽。

I saw ducks swimming next to me.
我 鸭子游泳 比邻 余 我看见我的旁边许多鸭子也在游泳。

这是一个美丽的一天

The sun is shining brightly
艳阳高照 明亮地 This can be construed as a poetic line which can be translated as "Beautiful is the sun shining high, brightening the earth. 太陽在明照

I decided to go for a swim in the lake.
我决定在湖中 去游泳 在湖

The water was cold, but refreshing.
水是冷 or 水是冷的 却 or 水凉爽 In a poetic form, 水寒清涼

I saw ducks swimming next to me.
我 看到鸭子在我身傍游泳 比邻 余

The above are just direct translations.  If written as a short paragraph, it is better to write something like, 

艳阳高照明亮地, 我决定在湖中去游泳.水寒清凉.小鴨子在身傍游. or 太陽高照所以决定去湖中游泳.湖水冷冰冰但覺得清爽愉快.小小的鴨在我的身辺游來游去.

这是一个美丽的一天 (一天 means one day,so needn't use 一个,一个means "one",and if we want to express "one day",we say “一天”not“一个天”)

The sun is shining brightly
艳阳高照 明亮地 (艳阳高照 is a set phrase,means sun shine is very good...the weather is very good,we can use 阳光灿烂 or 阳光明媚，is more oral.) 

I decided to go for a swim in the lake.
我决定 去游泳 在湖  （我决定去湖里游泳。“在湖” indicate a place,the place is 湖lake,I swim in the lake,so we say "I swim 在湖里"，but chinese say "I 在湖里 swim" .And 我决定在湖里游泳 shows a condition，我决定去湖里游泳 shows a activity.）

The water was cold, but refreshing.
水是冷 or 水是冷的 却 or 水凉爽 （凉爽 means cold but refreshing,I guess you don't want to express freeze,freeze means 冷！）

I saw ducks swimming next to me.
我 鸭子游泳 比邻 余(ancient Chinese)（next to me 在我旁边/附近，next to 在……旁边/附近）（I saw ducks swimming 我看见鸭子们在游泳。ducks means 鸭子们，“们” means plural.）

I saw ducks swimming next to me.

我看见鸭子们在我旁边游泳。

这是一个美丽的一天

The sun is shining brightly
艳阳高照 明亮地 真是阳光明媚的一天

I decided to go for a swim in the lake.
我决定 去游泳 在湖 湖里游泳

The water was cold, but refreshing.
水是冷 or 水是冷的 却 or 水凉爽 湖水虽然有点冰冷，但是却令人神清气爽。

I saw ducks swimming next to me.
我 鸭子游泳 比邻 余我看到有只鸭子在跟着我游泳

这是一个美丽的一天