The reasons it's unlikely that
Melkor had any minions of power worth speaking of in the sea are, to my mind, as follows:-
First and most obvious,
Tolkien never mentions them, implying that there are none worth speaking of.
Second, the sea was under the dominion of
Ulmo (a
Vala whose power rivalled that of
Manwe) and at least two who were counted very strong among the
Maia in Uinen and Osse. The fact that very few of the Maia are even named, let alone spoken of at the length these two are suggests they were mighty among the
Maiar. It seems three
spirits of such power could marshall the oceans pretty well between them.
Thirdly, Melkor didn't really have a need of sea
creatures. His war with the
Noldor and co. was essentially a defensive one seeing as in large part the idea was that he would hold onto
the Silmarils. Another big point of the war was that he didn't much like
elves and wanted to kill as many of them as possible. There was no real need, in this context, for Melkor to have a navy of any sort since most of the elves he wanted to kill were on middlearth anyway and didnt have any supplies or reenforcements being shipped in from
Aman that he would need to cut off.
Basically, any spirits that might be mighty enough to have withstood Ulmo, Osse and Uinen and held its own in the sea would have served Melkor better on land....