Code is providing a row of data but not formatting it as table

Question

It’s not displayed as table, because you don’t return correct HTML code. Let’s take a look at your code (I’ve put comments at end of incorrect lines):

$content="<table>";
$content .= '</tr><th>Week Beginning</th><th>Monday</th><th>Tuesday</th><th>Wednesday</th><th>Thursday</th><th>Friday</th><th>Saturday</th><th>Sunday</th><th>Weekly Total</th><th>Previous Week Totals</th></tr>';  // <- here you open TR with </tr>, so it's already incorrect
$results = $seconddb->get_results( ' SELECT * FROM seconddb_PontoonBoats_LineOne_2Log' );
foreach ( $results AS $row ) {
    $content="<tr>";  // <- here you overwrite previous value of $content with string '<tr>'
    // Modify these to match the database structure
    $content .= '<td>' . $row->{'Week Beginning'} . '</td>';
    $content .= '<td>' . $row->Monday . '</td>';
    $content .= '<td>' . $row->Tuesday . '</td>';
    $content .= '<td>' . $row->Wednesday . '</td>';
    $content .= '<td>' . $row->Thursday . '</td>';
    $content .= '<td>' . $row->Friday . '</td>';
    $content .= '<td>' . $row->Saturday . '</td>';
    $content .= '<td>' . $row->Sunday . '</td>';
    $content .= '<td>' . $row->{'Weekly Total'} . '</td>';
    $content .= '<td>' . $row->{'Previous Week Totals'} . '</td>';
    $content .= '</tr>';
}
$content .= '</table>';

So your function returns something like this, if there are no rows (it’s incorrect HTML):

<table>
    </tr><th>Week Beginning</th><th>Monday</th><th>Tuesday</th><th>Wednesday</th><th>Thursday</th><th>Friday</th><th>Saturday</th><th>Sunday</th><th>Weekly Total</th><th>Previous Week Totals</th></tr>
</table>

And something like this, if there are any rows found:

<tr>
    <td><VALUE FOR: $row->{'Week Beginning'}></td>
    <td><VALUE FOR: $row->Monday></td>
    <td><VALUE FOR: $row->Tuesday></td>
    <td><VALUE FOR: $row->Wednesday></td>
    <td><VALUE FOR: $row->Thursday></td>
    <td><VALUE FOR: $row->Friday></td>
    <td><VALUE FOR: $row->Saturday></td>
    <td><VALUE FOR: $row->Sunday></td>
    <td><VALUE FOR: $row->{'Weekly Total'}></td>
    <td><VALUE FOR: $row->{'Previous Week Totals'}></td>
  </tr>
</table>

And here’s the fixed version of that function:

function pontoon_table_shortcode( $args ) {

    global $seconddb;
    // Shortcodes RETURN content, so store in a variable to return
    $content="<table>";
    $content .= '<tr><th>Week Beginning</th><th>Monday</th><th>Tuesday</th><th>Wednesday</th><th>Thursday</th><th>Friday</th><th>Saturday</th><th>Sunday</th><th>Weekly Total</th><th>Previous Week Totals</th></tr>';
    $results = $seconddb->get_results( ' SELECT * FROM seconddb_PontoonBoats_LineOne_2Log' );
    foreach ( $results AS $row ) {
        $content .= '<tr>';
        // Modify these to match the database structure
        $content .= '<td>' . $row->{'Week Beginning'} . '</td>';
        $content .= '<td>' . $row->Monday . '</td>';
        $content .= '<td>' . $row->Tuesday . '</td>';
        $content .= '<td>' . $row->Wednesday . '</td>';
        $content .= '<td>' . $row->Thursday . '</td>';
        $content .= '<td>' . $row->Friday . '</td>';
        $content .= '<td>' . $row->Saturday . '</td>';
        $content .= '<td>' . $row->Sunday . '</td>';
        $content .= '<td>' . $row->{'Weekly Total'} . '</td>';
        $content .= '<td>' . $row->{'Previous Week Totals'} . '</td>';
        $content .= '</tr>';
    }
    $content .= '</table>';

    // return the table
    return $content;
}