The /ð/ sound is spelled with <th>. The /s/ sound is spelled with <s>, <ss> or <ce>. The homograph sow makes pairs with though and thou.
This is a contrast between a voiced dental fricative and a voiceless sibilant, close together in the mouth. It is not a problem.
Thanks to the rarity of the /ð/ spund, the mean density value is very low at 0.2%. The list makes 30 semantic distinctions, a loading of 83%.
bathe base bathing basing bathed based bather baser bathe bass clothe close lathe lace leather lesser lithe lice Louth louse Lytham lissom mouth mouse mouthing mousing mouthed moused neither nicer seethe cease seething ceasing seethed ceased that sat thee C thee sea thee see thence sense these C's these seas these sees they say thine sign though sew though so though sow those sows thou sow thus suss thy sigh writhe rice
John Higgins, Shaftesbury, November 2010.