In the past three years that I have been writing programs for fellow SUP paddlers I've played around with different training zones.
While it isn't too big an issue while on your own it is an issue when following a program or if you are training in a group. Basically you want to get the most out of the session and for the group to remain somewhat close to each other. To get the most out of your training you need to use each of these zones at various times. For speed work you'll be using more of Zone 5 and then Zone 1 and then for endurance work you'll be using more of those middle zones, it all depends on what race it is you're training for and what it is you want to work on.
While I believe a heart rate monitor is the best way to figure out your true training zones, personally I find it a bit tricky to look at my watch while paddling. Another way to train is by using a pace, again it requires looking at a screen while paddling but it also varies a lot in paddling depending on the tide, wind and depth of water. The guide below is based more on feel and listening to your body rather than looking at your watch for your heart rate or speed. I find it's pretty useful so have a read a let me know what you think.
REST (0% Effort): not padding at all
Zone 1 (20-35% Effort): effort we use at the start of a warm up or during an active rest period in-between sets. Tapping along without putting much effort into each stroke. You can easily talk and should not be sweating.
Zone 2 (40-55% Effort): effort we use at the end of a warm up, starting to get a bit of glide but can still hold a conversation and you should not be out of breath.
Zone 3 (60-75% Effort): Long and strong, our most efficient stroke. Can talk but not hold a conversation. Right on the edge of sweating and breathing heavily.
Zone 4 (80-95% Effort): Long and strong, with a bit more effort. Can not talk. Squeezing on each catch and beginning to sweat and breathing rhythmically.
Zone 5 (95-100% Effort) : Short and Sharp stroke, this is an all out sprint. Heart rate and cadence should be right up. Sweating and out of breath at the end of each effort. Focusing on the front end of the catch and abbreviating the power phase of your stroke.