"How do I know I'm working hard enough in the water? I have been told to swim aerobic pace but what does that mean if I cannot wear a heart rate monitor in the water?"
In the days before a lot of technical testing tools, my coach had me count heart rate after sets. It works like this.
1) take heart rate; start with a zero, count for 6 seconds
2) heart rate should be around 15 to 16 count
Main Set: (the set you are really there to do)
1) take heart rate; same as above
2) heart rate should be around 18 to 19 count
Here's why the pool can give you different numbers than on your bike (my lowest numbers) or run. The water cools your body down faster than air. So you may be busting tail in the water but you are loosing heat rapidly too. (Think of running in 40-50 F vs 75 or 80 F.) Second, the pressure of the water around you helps circulate blood making it a bit easier to hold a higher heart rate.
So for me, a long aerobic swim set should end with my heart rate at an 18 count for 6 seconds. If I am 17 or 16...I didn't push hard enough. This could be a result of not enough strength, fatigue or not pushing hard enough. Assuming all systems are go, you need to hit 18. Notice I'm not referring to time at all. Time on your swims for any distance will just be a product of strength x your heart rate = your time (what you can do with your current fitness level).
For example, If I swim 20 x 100m on 1:30 (my benchmark set) and my goal is 1:05 or better for all 20; if I am coming in at 1:07 or 1:06 and my heart rate is a 17 count - I need to push harder. This will take some experimentation on your part but you'll see the benefit (and the difference) soon. If I am coming in at 1:03 and :04 and have a 17 count...I need to push harder also. Perhaps I should be doing this benchmark set on 1:20 send off instead of 1:30. So as you get faster, you can try to hold the faster times longer. If Matt swims :57 for each 100 on 1:30 but he is at 19 count for 6 seconds...that is about right. The set takes on a different quality for Matt because he is getting 33 seconds rest instead of my 13 seconds of rest. Do you follow?
Swimming is hard work. Knowing how hard you are really working will help you develop quicker. Focus on your swimming fitness and power first...a few times a week work on your technique. On the main sets you should concentrate on form AND your times/heart rate. This is why most swimmers are smart folks in school. They are concentrating on multiple things at the same time. Think of a drummer keeping multiple sounds and pace of the song.
What do you hear in your head when you swim? Are you thinking of dinner? A song? Great swimmers are like a great band; many things happening at the same time to make a great sound. A good swimmer first thinks about their technique (repeatedly) during a set, then pace and effort, then a song. If you can't do all three at once...start with technique. For me on freestyle? Finish and roll. Finish and roll. Under that is my kick "thought" - kick, kick, kick, kick - BREATHE! Your kick is critical. Most triathletes have miserable kicking skills and it shows. On top of that is my internal pace clock. (to self) Ok, that last 50 was 31 seconds and if felt about like this. Calculate in fatigue and I have to pull a bit harder. Come on! The next layer is perhaps a song or my motivation. In high school it was the announcer giving splits out and saying who was winning. "Lane 3, Mitera turned at 28 seconds in first place and is on pace to win the Sectional Championship!" (Every rep. Every set. Every day. Every year.) Now it is more like, Phil Liggett, "Bob Mitera cuts a path through the slower pro swimmers as he heads toward the Ironman Hawaii swimming record. He has made up the difference between the age group and pro start and looks strong. His old training mate, Lars Jorgensen set the record in 1983 in 43:11. Won't Lars be surprised if he can do it!" Lastly, a song that keeps your rythm. A jammin' ass kicking song. Lately, it has been "Who Made Who" by AC/DC.
The club team I was on had only two rules. 1) be on time 2) hold the heart rate you were told to hold or the entire team starts the set over. The top priority is heart rate. If you aren't working at that level, you are wasting time in the pool going too easy. Do you want to be the reason that 60 people start a set over? Didn't think so.
A word about pace. Being a triathlete and needing to sight. I have gotten into a habit of lifting my head at each 50 to view the clock. This helps me for two purposes. 1) I know my pace and can adjust it on the next 50. 2) It forces me to lift my head up high enough and long enough for me to read the clock. Not unlike looking for the ever elusive turn buoys. When I coach, I've been known to move the clock around to force people into thinking about one more thing: where is the clock? Just like in an open water race - where is the next buoy?
Master this and you'll be smoke on the water.