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First, Tai languages do not seem to have a native word for "sea". The Thai and Lao words for sea are ทะเล tha.le: and ທະເລ (ทะเล) tha.lé: respectively, which are borrowed from Khmer tʊənlee meaning "river." The Shan word for "sea" is ပၢင်ႇလၢႆႇ (ป่างหล่าย) pàang.làai which is borrowed from Burmese
ပင်လယ် pin.le (written pang.lai). And the Tai Dam word for "sea" is nɔ̌ng.lǔang (หนองหลวง), which literally means "great lake or great pond."
Next, it does not seem like Tai languages have a native word for "shark". The Thai and Lao words for shark are ฉลาม cha.lǎam and ສະຫຼາມ (สลาม) sa.lǎam respectively, which are borrowed from Khmer claam. The Shan word for "shark" is ပႃငမၢၼ်း (ปลาง้ามาน) pǎa.ngâ.máan which is borrowed from Burmese
Thus, the fact that these SW languages lack native words related to sea may suggest that Tai people's original homeland was not near a sea.