We can use this XSLT step to find the low and high tides and set them as attributes:

<p:xslt>
  <xsl:transform version="1.0">

  <xsl:template match="t:tideinfo">
  <xsl:variable name="today" select="string(@today)"/>
  <xsl:copy>
    <xsl:attribute name="low-at">
       <xsl:for-each select="t:tide-data/t:tide-level" mode="first">
          <xsl:sort data-type="number" order="ascending" select="@level"/>
          <xsl:if test="position()=1">
          <xsl:value-of select="@time"/>
          </xsl:if>
       </xsl:for-each>
    </xsl:attribute>
    <xsl:attribute name="high-at">
       <xsl:for-each select="t:tide-data/t:tide-level" mode="first">
          <xsl:sort data-type="number" order="descending" select="@level"/>
          <xsl:if test="position()=1">
          <xsl:value-of select="@time"/>
          </xsl:if>
       </xsl:for-each>
    </xsl:attribute>
    <xsl:copy-of select="@*|node()"/>
  </xsl:copy>
  </xsl:template>

  <xsl:template match="t:tide-level[1]" mode="first">
  <xsl:value-of select="@time"/>
  </xsl:template>
  
  <xsl:template match="t:tide-level" mode="first"/>
  
  </xsl:transform>
</p:xslt>