If you are discussing with someone on an island, the clarity of the water, then due to your surroundings, it is neither necessary to mention the ocean nor the island: 'here' or 'it' or 'this water' is sufficient.
If something is 'a first' for someone, then it is a new experience.
I've never seen such clear water!
こんなに透明な水は見たことがありません！
It's a first for me to see such clear water.
こんなに透明な水を見るのは初めてです。
島の人と、水の透明度について話しているとき、oceanやislandと言う必要はなく、here（ここ）、it(それ）、this water(この水）という言葉を使うといいでしょう。
'a first' とは、初めての経験という意味です。
The ocean around this island is the most transparent I've ever seen.
The ocean around this island is the most crystal clear I've ever seen.
「この島の海は」を英訳しますと "the ocean around this island" 又は "the sea around this island" （どちらも文字通りですと 「この島を囲まる海」）になります。
透明は "transparent" なんですが、この文脈で "crystal clear" （結晶のように透明というニュアンス） の方は自然な言い方に聞こえると思います。
少し文章を変えますと "This island has the most crystal clear ocean I've ever seen." という言い方でも伝われます。
ご参考になれば幸いです。
"The water here is crystal clear." To say that something is crystal clear means that you can see all the way through it.
"This is the clearest water I have ever seen." To tell the listener that something is the clearest would mean that you cant compare it to something else.
"The water here is crystal clear."
ここの水は、透き通っている。
crystal clearとは、向こうまでずっと見える、透き通っているという意味です。
"This is the clearest water I have ever seen."
こんなに透き通っている水みたことない。
the clearestとは、他に比べることができないくらい、その水が透き通っているという意味です。
Likening the water to something that is known to be clear is a good way of describing it, so you could could use the descriptions 'as clear as glass' or 'crystal clear'.
To express that it is the clearest you have ever seen, you could also say
'I've never seen such clear water!'
'This is the first time I've seen water so clear'
みんなが知っている透明なものに例えて言うといいと思います。例えば：
'as clear as glass'（ガラスのように透明な）
'crystal clear'（水晶のように澄み切った）
今まで見た中で一番透き通っていると表したいなら、以下の表現も使えます。
'I've never seen such clear water!'
これほど透き通った水は見たことがない！
'This is the first time I've seen water so clear'
これほど透き通った水は見たことがない。
The ocean on this island is the clearest sea water I've ever seen
The water in [*name of the beach*] beach is the clearest I have ever seen.
You can say it in the way shown above. E.g. The water in Camps Bay beach is the clearest sea water I have ever seen. You can also use the word "transparent" instead of the word "clearest".
上記のように言うことができます。
例： The water in Camps Bay beach is the clearest sea water I have ever seen.
キャンプスベイの水は今まで見た海水で一番透き通っている。
"clearest" の代わりに "transparent" を使うこともできます。
The ocean around this island is the most crystal clear i've ever seen
I've never seen an ocean as crystal clear as this one
The ocean around here is the most transparent i have ever seen
If you are wanting to explain that the sea/ocean is very clear then you can use the term 'crystal clear' which means the same as 'transparent' or you can see right thorugh it.
To add ive ever seen means you have not seen the ocean/sea as clear as this before
海がとてもきれいだったことを説明したい場合、「透き通った」と言う意味の”crystal clear"というフレーズを使って表現出来ます。
”I've never seen"を付け加えると、「こんなに透き通った海を見たことがない」という意味になります。
Translucent is a word that helps us describe things that we can see through, or that are clear. It shows us that light can travel through the object, making it bright and clear, or easy to see. For example: The water is translucent, we can see right through it!
translucent'は透き通った物を表す時に使えます。光を通すことのできる、明るくて透き通った物を表します。
例文
The water is translucent, we can see right through it!
（水が透き通っている、中まで見えるよ）
1. This is the clearest sea water I have ever seen in my life.
2. The sea water around this island was the clearest I have ever seen in my life.
1. If you are actually there and directly referring to this body of water this would be a concise way of saying it. "Sea water" generally refers to ocean water (or salty water whether in a sea or the ocean) and is most commonly used in North American English.
2. If you are not on site, and referring to a past experience the second sentence would be more precise.
イギリス 
アメリカ合衆国 
南アフリカ 
