氵 (water radical, left-side form of 水) anchors the original image: 涉 literally meant to wade across a stream, feet (步) splashing through water. The radical is essential — without it, 步 alone just means a step on dry ground.
步 (step) supplies the second semantic half. 步 itself depicts two feet, 止 over 止, walking. Combined with water, the picture is unambiguous: two feet wading. The meaning then broadened — to ford, to experience, to involve oneself in (涉及, 涉嫌).
