Gamma (tenses)

首先要更正你一個想法：(an/the) animal is beautiful是你主觀的感受，並不是永恆不變或真理的表達。

第二
如果這句是敘述或記錄你的一次旅程或參觀的經歷...這假定是因為句子是用past tense作句，那麼當中的描寫/形容/描述(句括你感受)都應該是「記綠」「當時」的情形(當然你的想法是我現在仍是這想法或感受，但注意你的文章是有時空範圍的)。


基於以上兩點，無論你對那動物beautiful的感情有多永恆，正確的寫法是
I was amazed that the sea animals were so beautiful.


你如果多看(英語)小說你便會見到這類看似表達當時情況但卻以過去式寫句子的句式，甚至更永恆真理或事實不變情節或主題都是會以past tense組成的。另外附加提另有一類寫作亦是全past tense，就是寫工作/習作/實驗等等的報告。例如就算那他化學反應或實驗內容是你的發明或當時的發現有幾震撼的現場感，但報告格式就是past tense。

盼望在你的誤解了英語present tense和past tense用法意識未入心前快快更正。真實用tense不是像上英文堂咁簡單的。

I was amazed that the sea animals were so beautiful (when I visited Great Barrier Reef.)

Grammar:-
I was amazed that the sea animals are so beautiful.
Your analysis are the present fact are correct.
are verb intransitive, is present indicative plural of be,indicating animals are beautiful.
ie:- They are beautiful.
Were it not for the subjunctive,past tense of are=were should not be used, as it were; so to speak in" were so beautiful."

I was amazed that the sea animals were so beautiful --correct (或者以前覺得很美，而現在不再覺得美)

if you want to use present tense, should be
I AM amazed that sea animals are so beautiful. (不能夠一個句字內又有過去式又有現在式)

Your analysis is correct.
I was amazed that the sea animals are so beautiful. - This is the right sentence.