Which is better for swimming and diving?

The Pro Diver 9204OB. Its 200m rating and screw-down style construction suit recreational diving, while the I-Force 1512 is rated for swimming at 100m.

Which one lasts longer?

The Pro Diver 9204OB, mainly because of the sapphire crystal. Mineral glass on the I-Force picks up scratches that sapphire shrugs off.

Is the chronograph worth stepping up to?

Only if you use a stopwatch. The I-Force 1512 adds subdials and a date, but you give up sapphire glass and half the water resistance.

The Pro Diver 9204OB. The I-Force chronograph has a thicker case that sits taller on the wrist.

Yes. Both show the date, but only the I-Force 1512 adds chronograph subdials.

Can I change the strap on either?

The I-Force 1512 is commonly sold on steel, leather or canvas, so swapping is easy. The Pro Diver 9204OB comes on a steel bracelet with standard lugs, so aftermarket straps fit.

Invicta I-Force Chronograph 1512 vs Invicta Pro Diver 9204OB

Buy the Pro Diver 9204OB if you want a dive watch that holds up: sapphire crystal, 200m water resistance and a stainless steel bracelet make it the tougher, simpler watch. Buy the I-Force 1512 if you want a chronograph with a date and a more aggressive look, and you accept mineral glass and a busier dial. Owners of both report solid value for an entry-level Invicta; the Pro Diver is the one most people keep wearing.

Choose the I-Force 1512 if you want a chronograph with subdials and a date on a rugged-looking case.
Choose the Pro Diver 9204OB if you want a clean dive watch with sapphire glass and 200m water resistance.

Prices

Invicta I-Force Chronograph 1512: $51.00 at Jomashop (checked today)
Invicta Pro Diver 9204OB: $63.00 at Newegg (checked today)
Difference: the Invicta I-Force Chronograph 1512 is $12.00 cheaper than the Invicta Pro Diver 9204OB today