Maybe I'm getting to far ahead of myself here, but to expound on Wiglafs' excellent proposal, The moon would of course effect tides, but instead of worrying about distance and size, you can make your moon any size within reason, as mass effects gravity, thus allowing you to create maybe 2 moons, one on a fast orbit the other on a slower orbit, this would allow for many high tides within a month and one exceptionally high tide and one exceptionally low tide per month, this can create the feeling of urgency, if anything needs to be done within those time frames.
But back to your ambushing predator, Is it magical, id est, does it have any magical means of defense or attack or is it non magical?
Also, what do these mammals hunt, as this would give you a better idea of how this creature would attack.
Does it have a proffered prey, this would mean it evolved in a specialist way, having tricks or inherent knowledge on how to best get a meal.
Maybe it is a scavenger or an opportunist, what ever you decide its prey to be, it would be best to have that prey as a land/tree dweller, as this would give plausible reason for the creature to have the ability to launch attacks up and out of the water.
Other factors to be considered, can it chew? or does it have to roll to strip flesh from bone?
does it give birth in water or on land, the latter would allow for you to have the creature survive out of water for periods, unlike dolphin/whale type mammals.
A huge otter type creature would be most successful i would imagine, changing the jaws and features as you feel are appropriate for the overall look of your creature.