There's no universal word for 'sea' in IE languages. The most widespread (Celtic, Latin, Germanic, Balto-Slavic) one is *mori-, related to Altaic *mju:ri 'water'.
Another widespread root is *seH2l- 'salt, sea'1 > Greek halê 'sea'2, which I think ultimately comes from PNC *q’eɦlV (˜ -ɫ-) 'bitter'3, with an evolution PNC *q’- > pre-PIE *χ- > PIE *s- (by Fournet's Law). This Vasco-Caucasian root is reflected in Basque gatz 'salt', gazi 'salty' + PNC *ts’s’wenhV 'salt'4 > *gas-dane > gazta(i), gazna 'cheese', both Cantabrian loanwords.
Dolgopolsky proposes a Paleo-Eurasian root *dalqV 'wave' (ND 526) reflected in IE *dhelH- > Greek thálassa (Attic thálatta) 'sea'. In a IE substrate language (probably Italoid), this root gives Greek sálos 'turbulent movement of the sea, flushing of the waves', Latin salum 'open sea; sea waves'5.
_____________Another widespread root is *seH2l- 'salt, sea'1 > Greek halê 'sea'2, which I think ultimately comes from PNC *q’eɦlV (˜ -ɫ-) 'bitter'3, with an evolution PNC *q’- > pre-PIE *χ- > PIE *s- (by Fournet's Law). This Vasco-Caucasian root is reflected in Basque gatz 'salt', gazi 'salty' + PNC *ts’s’wenhV 'salt'4 > *gas-dane > gazta(i), gazna 'cheese', both Cantabrian loanwords.
Dolgopolsky proposes a Paleo-Eurasian root *dalqV 'wave' (ND 526) reflected in IE *dhelH- > Greek thálassa (Attic thálatta) 'sea'. In a IE substrate language (probably Italoid), this root gives Greek sálos 'turbulent movement of the sea, flushing of the waves', Latin salum 'open sea; sea waves'5.
1 Without any linguistic evidence, Arnaud Fournet relates this to Kartvelian *zoɣw-. See his document.
2 A feminine word whose masculine counterpart is halós 'salt'.
2 A feminine word whose masculine counterpart is halós 'salt'.
3 The glottalized stop is retained (with secondary labialization) in Kartvelian *q’wel- 'cheese'.
4 Native outputs from this root are itsaso 'sea', itze 'sea' (an archaic form quoted by Trombetti) + urde 'pig' > izurde, gizaurde 'dolphin' (lit. 'sea-pig').
5 Which again Fournet unjustifiedly relates to IE *seH2l-.
4 Native outputs from this root are itsaso 'sea', itze 'sea' (an archaic form quoted by Trombetti) + urde 'pig' > izurde, gizaurde 'dolphin' (lit. 'sea-pig').
5 Which again Fournet unjustifiedly relates to IE *seH2l-.
