Who told you your head was too submerged??? Too submerged relative to what 'correct' position??? What explanation was offered???
Be very wary of any advice about the 'right' way to swim that is not backed up by a solid argument.
The traditional argument is that the head should be slightly raised so the waterline is at your forehead. I have come across two justifications for this -
1. To see ahead
2. It lowers your feet slightly to stop them splashing the surface, which is less propulsive.
[Actually there is a third common 'argument': "I don't know why, it just is; that's what I was taught." I think we can safely ignore this one!]
The counter-arguments (and the TI view as I understand it are) -
1. You don't need to look ahead when you are trying to swim as ergonomically & streamlined as possible. In the pool there are marks on the bottom for that very reason. In open water, you can take a sighting every 15 strokes or so, or 'cheat' by just staying in the same direction as the swimmers to either side of you.
2. Raising your head does indeed lower your feet, which is a BAD thing because it increases drag. This idea of avoiding surface splash only applies to the out-of-date style of swimming flat on your front. When you swim with rotation, your body position means that this is simply not a problem.
Therefore you should swim with your head aligned neutrally with your spine in order to achieve fore-and-aft balance which will keep you streamlined.
BUT if you are burying your head with your chin on your chest, then you are too submerged, and this will create a whole load of other problems.
So without seeing your actual head position, no one can say definitely whether the advice is good or bad. But I would bet a lot of money that the advice is from a traditionalist - and best ignored.