If the sound you're looking for is soft then hush could work. Perhaps her partner was chattering away and Anna wanted the person to be quiet. If Anna was tired, bored, or even feeling romantic, then sigh could equally fit. The word look could be said sotto voce in a single long breath, it would have an almost breathless quality to it, in this case, sigh would not be out of place.
“Look,” hushed Anna, pointing forward
“Look,” sighed Anna, pointing forward.
If instead you are looking for a more urgent tone, one that expresses a threat, tension or concern then either hiss or sibilate will do. They both express ways of speaking in a low voice.
“Look,” hissed Anna, pointing forward
“Look,” sibilated Anna, pointing forward