Display Data in Table from External Database in WP using Shortcodes

While it is VERY bad practice to have spaces in the object names, there are times where it may be out of your control. You can use the syntax of $object->{'Property Name'}, like this:

// add the shortcode [pontoon-table], tell WP which function to call
add_shortcode( 'pontoon-table', 'pontoon_table_shortcode' );

// this function generates the shortcode output
function pontoon_table_shortcode( $args ) {

    global $wpdb;
    // 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>Previosu Week Total</th></tr>';
    $results = $wpdb->get_results( ' SELECT * FROM wpdb_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;
}

UPDATE:

It looks like you had $content="<tr>"; which resets the string instead of appending to it, changed that to $content .= '<tr>';