This should work. Wasn’t closed well
<div id="cooler-nav" class="navigation">
<div class="wpb_wrapper">
<h3>More Underwater Media</h3>
</div>
<?php $prevPost = get_previous_post(true);
if($prevPost) {?>
<div class="nav-box previous">
<div class="navig_thumb_wrapper">
<?php $prevthumbnail = get_the_post_thumbnail($prevPost->ID, array(100,100) );?>
<?php previous_post_link('%link',"$prevthumbnail", TRUE); ?>
</div>
<?php previous_post_link('%link',"<p>< %title</p>", TRUE); ?>
</div>
<?php } ?>
<?php $nextPost = get_next_post(true);
if($nextPost) {?>
<div class="nav-box next" style="float:right;">
<div class="navig_thumb_wrapper">
<?php $nextthumbnail = get_the_post_thumbnail($nextPost->ID, array(100,100) );?>
<?php next_post_link('%link',"$nextthumbnail", TRUE); ?>
</div>
<?php next_post_link('%link',"<p>%title ></p>", TRUE); ?>
</div>
<?php } ?>
</div>