Why was a Plimsoll symbol chosen to indicate standard state?
Prigogine's Chemical Thermodynamics gives a bit more context as to why a plimsoll symbol is somewhat more preferable and is indeed used to denote non-zero nature of standard values [1, pp. xxiv, 86]: 


  The properties of a pure substance are denoted by a superscript $^0$. The use of this symbol to denote "standard" thermodynamic quantities is then permissible only when these refer to the pure substance as standard state. Some other convention is needed to denote the more general "standard quantities" defined in equation (7.51), where the standard state may for example refer to an infinitely dilute solution. After much consideration the symbol $^⦵$ has been introduced. This symbol is based on the circle, and is thus closely related to the more common (but occasionally confusing) notation for standard quantities. It seems useful, however, to associate it with the plimsoll mark which, appropriately enough, refers to a reference state of loading of a ship; this use of an ideogram has, we believe, some value in keeping the essential nature of "standard states" in prominence.
  
  Quantities denoted by the plimsoll symbol are still, in general, 
  functions of temperature and pressure; the idea of "standardization" 
  of temperature and pressure is not stressed. When necessary we denote 
  quantities at the standard pressure (which for convenience is nearly 
  always chosen as the unit of pressure) by the superscript $^†$, quantities at a standard volume (which for convenience is always chosen as the unit of volume) are denoted by $^‡$. Quantities referring to a (usually hypothetical) ideal system are denoted by $^\mathrm{id}$.
  
  [...]
  
  $$P = P^⦵(T,p) + P^M(T,p,x_1\ldots x_c) \tag{7.51}$$


It's also interesting that Lewis and Randall, known for defining the standard state in the beginning of XX century as we know it, used an upper circle [2, p.623]:


  $G^\circ$ A property of a substance in its standard state.  


However, they referred to it as to a standard reference state (distinguished from a standard state) throughout the textbook [2, p. 88, 99]:


  While we cannot give the absolute magnitude of $\bar{H}$ for a substance in solution, we may ascertain how much greater or less this is than the heat content of the same substance in some chosen state. Thus, at any temperature, if we are dealing with the partial molal heat content of water in some solution in which water is the solvent, we may choose pure liquid water as the reference state, and denote its molal heat content by $H_1^\circ$.
  
  [...]
  
  It is highly convenient, especially for purposes of concise tabulation, to know the heats of reaction when various substances are formed from their elements. It is therefore desirable to choose some one standard reference state for each element. For this purpose, we shall, at all temperatures, take the element at a pressure of one atmosphere, and in that form which is most stable or most common at room temperature. Thus liquid mercury, gaseous oxygen, solid iodine, rhombic 
  sulfur and graphitic carbon, all under a pressure of one atmos­phere, will be considered to be in their standard reference state.


References


Prigogine, I.; Defay, R. Chemical Thermodynamics; Treatise On Thermodynamics; Longmans, Green &amp; Co Ltd: Glasgow, 1954; Vol. 1.  
Lewis, G. N.; Randall, M. Thermodynamics and the Free Energy of Chemical Substances, First Edition later Printing edition.; McGraw-Hill Book Company, 1923.