@Jeansoo - Glad you were able to get the lifeguard career working in another world. :)
Making the water shallower can be difficult to get exactly right because it has to be the proper depth for Sims to play in the water as well as large enough to perform the interaction. Looking at your second screenshot, I would start with the terrain in the left hand corner where the water is most shallow and level it across the rest of the shoreline, then raise or lower it in small increments as needed after testing the height (knee-high or so on an adult Sim), and then soften the terrain to blend.
For seeing underwater, you will need to hit the Q key several times in camera mode (or just hold it down). If the camera stops and won’t go below the surface of the water, you may be too close to the Sim(s) in the ocean; try from further away. A good place to start is looking out from the shallow water at the shoreline.
It looks like your Sims were very busy and weren’t stuck at an empty beach this time. :) Sometimes multiple Sims will need rescuing or CPR at the same time like that, and you might have a hard time keeping up. When that happens, I set NRaas Relativity to slow time down a bit so my lifeguard Sim can get to them all, particularly if I’m playing the Seaside Savior LTW. On other days, I’ll throw a party at the beach where my Sim is working so more Sims show up. xD
