If the aquanaut is facing the enemy, they will tend to get hit in the front, but will frequently get hit on either side. Only their back armor will not be hit-able. You can mod the armor facings if you don't like such weak sides making your front vulnerable.
The soldier's hitbox is a cylinder divided into four quadrants. If a soldier faces their diagonal toward the enemy, such that exactly two quadrants are visible to the enemy, then the odds of hitting either of the two far quadrants is drastically reduced if not eliminated entirely.
Some mods add a left-side shield to soldiers, so that they have especially high front and left armor. Those units will have very sturdy and reliable defense when properly facing the enemy, but it takes up the left-hand slot so that they cannot use a two-hand weapon with full accuracy.
I don't know all of the details, but you can harvest the work from other mods which have done it, listed here:
Re: [Q] Riot Shield mod for Open XCOM Extended?- - -
Under armor only applies to explosive-type attacks (such as HE, freeze) which detonate either under or adjacent to the soldier. If the center of the blast is any farther than that, it will also hit side armor.