Not that you want your weight-training program to get too specific, but what strokes do you swim?
Assuming already efficient technique, one way to shave a few seconds off your time is to develop a faster, stronger, larger start off the blocks and turn off the walls. Developing leg and hip strength is going to help this and it’s going to be similar to training for a higher vertical. Squats and deadlifts focusing on strength/power and, perhaps, a dumbed down Olympic pull (jump shrug or high pull from the floor) and some jump squats.
Beyond that, you want to focus on big compound exercises and strength endurance. Even in your fastest races, you’re still going to be operating at nearly full power for a lot longer than than the few seconds it takes to pull a single, double, or triple. I’d recommend loading up a heavy weight as possible and repping it for 30-60 seconds on the clock. Work on density of work by manipulating set lengths and rest periods. This is going to have a big effect on your muscles and, consequently, your cardiovascular system. It’s going to be hard work, but it’s going to improve all the right things.
I swam competitively at a low level a few years back and I can say with confidence that my own team as well as the teams I was competing against had dryland programs from the stone age. They were basically indistinguishable from a rank beginner’s or, worse, a bodybuilder’s workout. Your current program is also unfortunately a combination of both.
Ultimately, remember that it’s the work you do in the water that is going to have the biggest impact on your swimming. Work your ass off in the pool and, in your spare time, keep refining your technique – strokes, breathing, starts, turns, touches, etc. Dryland conditioning is a great tool that can make a good swimmer better, but, by no means, will it make up for a lack of work in the water. With that in mind, also keep perspective about the intensity of your dryland versus what you’re doing in the water. Some of what I’ve suggested is pretty taxing work that is naturally going to have a detrimental effect on your recovery.
Adjust your program based on what phase of the competitive season you’re in and, while you shouldn’t be afraid of working hard both on land and sea, remember that it’s the time and, more important, performance in the water that always takes final priority.