Why would they make the suit so heavy?

actually, they could probably make out fairly well, if they were underwater and had two good legs.

as for why the suit is as heavy as it is, i imagine it has to be a certain weight to sink low enough, but beyond that, i don't know. just that they use so much material to make a proper suit, it ends up weighing a ton.

I suspect the suit was weightless under water. They said it weighed 290 pounds but it was much bulkier than a man and probably had twice the volume of a man meaning it displaced approximately 290 pounds of water. Hence it would be weightless.

well, the helmet and collar weighted about 125 pounds (in that scene he was wearing an extreme depth mixed gas variant, i'm not exactly sure how much it weighed but it had some extra equipment that allowed the diver to go deeper and thus weighed more than the standard mark V which was already quite heavy). The belt and boots added the rest of the weight because the helmet made the diver very top heavy and he would've been standing on his head when he hit the bottom.

When in the water a human weighs much less because they are composed of mostly water and that much of them is supported by the water they are in(look at the tag on a life jacket they're only designed to support like 20 pounds) In any case, with that in mind, it would still take a very strong person to use that getup, but in the water it was much easier to handle. And yes, MANY divers moved around on the sea floor like that.

I read somewhere that those type of heavy suits are made that way in order to resist strong currents. I'm not a diving expert but I remember reading that.

Although I wasn't a Navy Diver, I was a commercial Diver and during training school I had the opportunity on many occasions to dive the Mark V suit. They are extremely heavy and cumbersome topside, but underwater they are pretty easy to move around in and actually somewhat comfortable to wear. The suits are so heavy to counter the buoyancy when filled with air under water. The entire suit fills with air which helped keep the diver insulated and also provided a short, secondary air supply in case anything went wrong. Even the boots are weighted to prevent an inverted ascent where the Diver surfaces upside down.