If you're talking about split-second communication, like shouting, I think touch is your best bet - it is immediate, while something like writing or visual signs or gestures require the other person to be looking in the right direction, at the right time. Tapping at or thumping someone will get their attention immediately - the direct equivalent of shouting, since it actively draws the attention, not waits to be noticed. if someone needs to pay attention, being twapped will draw attention from the most distracted, if they need to move, being dragged out of the way is better than being run over. Most basic, urgent communication will have a touch translation anyway, since basic, urgent communication is very likely to have equivalents in every method of communication we can find, create, or imagine - the better to survive in any situation where they might be needed.
If someone's outside of physical range, slapping at something else, like a wall, or water surface, or instrument, will create sound to alert your humans, and create waves/motion/disturbance in the water or whatever the equivalent sense is that your other people use to alert themselves to non-obvious motion. Or use a flashlight or something, for communicating pay attention across the distance... it will be needed to grab attention in case someone is looking the other way when they really need to be listening.
For less urgent communication, some combination of sign language or gestures would probably be more versatile (and again, touch bridges that gap between what a person is looking at and what they need to "hear" right away). You can also involve writing or better yet, drawing for precise details once there's time for explaining. translation passbooks, machines, pidgin, or anything else can be developed or used at their leisure.