Diving Hard-Suit :
Diagram : http://img841.imageshack.us/img841/2103/divinghardsuit.png
Used for depths in excess of 100 feet for non-modified human (no depth plasmid or Big Daddy modifications)
Above that depth the conventional diving suits and scuba gear can be used (with the required timed ascent schedule because of the pressurization).
Typical Rapture Diving Hard-suits were sufficient for depths to about 1000 feet (useable for all city construction). The diver is kept at 1 atmosphere (suit holds out the water pressure) so diver may exit the suit immediately (no depressurization delay).
- Work Lights on chest (0.53% of the surface light reaches a depth of 100 meters (330 feet) )
- Whats that over the shoulder thingee ?? Spotlights or camera (or both). Probably not a gun.
- Air tanks to keep a diver going for many hours, if Oxygen for even longer.
- Utility recharge hookups (battery charging, air charging, fuel charging for hydraulics)
Weight ~850lb including the diver (to achieve near neutral buoyancy), hydraulic assist would be useful to move that mass. Pressure at 1000ft down is 31,3 atm (459psi) which takes thick steel construction and carefully fitted joints to keep the water pressure out. That special advanced material 'Ryanium' might be used, but the required structure is still bulky and cumbersome.
You can see why alot of the construction was done from the mini-subs and structural sub-assemblies were assembled near the surface (<100 feet on the shallow side of Mt Ryan) when possible to allow use of simpler conventional diving suits. Even with the thousand-odd construction workers much of the contruction was done with machinery (and Hard-Suit Diving is a specialized skill).
The construction scenes depicted in Ryan Amusements Rapture Memorial Museum are men wearing the conventional diving suits and thus are at a fairly shallow depth. The concept art had them wearing Big-Daddy-like hard-suits. http://bioshock.wikia.com/wiki/File:Ss_preview_bio214.jpg
Advanced deep hard-suits can protect the diver down to more than 3000 feet depth (Abyss depths), but are much bulkier (weigh more), complex (more maintenance), and have much more restrictive movement (example - claws instead of gloved hand for manual manipulations). One advantage to the bulky deep hard-suits is that you can pull your arms in to scratch your nose while wearing it.
Later Plasmid assisted divers may have been able to use the conventional diving suits(depth tonic) and simpler hardsuits without hydraulics (strength tonics) and possibly go to the Abyss depths without the complicated deep suits. But for the city's construction there were no ADAM enhancements yet existing.