There is a cage like structure surrounding the entrance to the underwater laboratory. At one point, the door is ripped off it's hinges by the monster. Later, when they are on the way to blow up the alien ship, the door of the cage has been replaced.
At 30:30 and 30:50, the same scene is repeated. The three divers all approach the same structure. Cdr. Wayne is leading and grabs the structure then moves on.
When the creature attacks the support ship captain, he is wearing boots. Later, as he is swimming, his feet are webbed. As he swims, you can see the outline of his air tank in the hump beneath his dorsal fin.
You can see the reflection of the diver's mask behind the creature's eye sockets.
At 31:25+ minutes it's obvious all three divers are standing on something prior to entering the unknown vessel, although Cdr. Wayne does try to make it look like he has to muscle his way up into the vessel.
Near the end of the movie, when Cmdr. Wayne and Maddox are leaving the underwater lab with dynamite, headed to the alien probe ship, a third diver is seen smack dab in the middle of the shot pushing their two-person mini propulsion craft.