Hydra is a difficult weapon to use in SR, and that's precisely why I like it. Once you learn to deal with its shortcomings, you will have a beast on your hands. There's a few things I do to help me use Hydra.
Keep your distance. Similar to what Calamaro said, you want to stay fairly far away from your enemies. You don't even need to have teammates clear enemies for you. If you stay far enough to have the time to get a full charge, mow down every stinking enemy in that general area. Though teammates are recommended for ferrying eggs.
Use partial charges. Waiting for a full charge to kill something will likely get you killed, or your teammates will have everything killed. The only issue is memorizing how much of a charge kills what. I'd say the safe bet is a half charge. At a half charge, you can kill a Steelhead, Steel Eel, Scrapper (if turned around), and a Cohock + a few Chums. I need to spend more time with it in Salmon Run to tell how much of a charge is needed to kill everything.
Jump and shoot at the ground. You are going to run into situations where you will be surrounded by sludge, and you won't have time to ink a path for yourself. So sometimes in order to get around, you will need to jump in squid form and tap shoot at the ground. Actually just tap shooting at the ground with Hydra is a valuable skill.
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For luring enemies, it's really situational for me. If all of us are far away from the basket, and a Scrapper or Maws spawns near us, I say kill it. If we already have eggs, then I'd rather ignore it and lure the thing to the basket. For Steel Eels, I kill 'em immediately. Those things can get frustrating if you let too many spawn. Especially in high tide. However if it's a Cohock charge and I'm playing with friends, I'll wait until the enemies are near the basket to kill the enemies (including Steel Eels). Randoms would just get frustrated with me and use another cannon.