It's most probably true that SEA SHANTIES was the very first Prog-Metal album. Actually before 1970 I can recall only bands like DEEP PURPLE, CREAM or BLUE CHEER, but none of them playing that heavy nor progressive in a way. MC5's "Kick Out The Jams" from the same year was probably the very first Metal album in general, but they didn't play progressive at all. Nevertheless just for that reason alone I'm hesitating to call it a masterpiece.
Futilist's Lament and Death Warmed Up are really a true devil's ride with extremely amped guitar and violin. At times HAWKWIND is coming to my mind. Quite heavy stuff, but for my ears and listening habits becoming a bit too monotonous and noisy after a while (the same problem I have with the heavy stuff of the mentioned band). A rather huge contrast when third track Pushed, But Not Forgotten is starting even very mellow, soon after amps are switched on again, but on this one the music is much different from before and of much bigger quality and versatility. Especially here and as well in Walkin Down Their Outlook in the more quiet sections Tony Hill's vocals remind very much to those of Jim Morrison. The violin is sounding in these and in the following tracks much better and less distorted than in the first two. Missing Out reminds a bit to CREAM at least in the beginning before violin comes in. The longest track of the album is a very heavy and very interesting versatile one as well. Nowhere is a bit less heavy featuring again nice vocals reminiscent of THE DOORS.