ハワイにはたくさんのきれいな海があるって英語でなんて言うの？

There are various ways of expressing that something is beautiful. You may choose from a variety of wonderful adjectives such as gorgeous, pretty, pleasing to the eye.
To suggest that there is a lot of something it helps to use words such as abundance, numerous, plenty of.

"There are beautiful beaches all over Hawaii." tells people that the beaches in Hawaii are pretty.

If there is a 'plethora' of something, it means there is a large or even excessive number of these things
Another way to explain a large quantity is to use the adjective: 'numerous'

If using the term many you mean there is a lot of so to say `there is many beautiful beaches in Hawaii' it means that there are a lot of beautiful beaches in Hawaii you could also use the words numerous/multiple or countless they all mean there is a lot of something
Beautiful means something looks nice/lovely and can be admired when looked at

There are many words you can use to describe something as beautiful. Some of these synonyms include: pretty, alluring, stunning, and magnificent.
And there are many synonyms for the word many. Some of which include: countless, numerous, and multiple.
So you can use any combination of these words. For example: Hawaii has numerous, stunning beaches. or Hawaii has a countless amount of alluring beaches.

The three sentences you see above are terrific ways to express to your listener that Hawaii has many beautiful beaches. In the last sentence you will notice the adjective breathtaking. This word means amazing or stunning. This is a word that is common in our everyday conversation and it would make a great addition to your vocabulary.

When you want to explain that Hawaii has many beautiful beaches, then you can say t he following:
-Hawaii has many beautiful beaches
-There are a number of beautiful beaches in Hawaii
-There are a lot of beautiful beaches in Hawaii